Dramatically Enhancing Oil Recovery via High-Efficient Re-Fracturing Horizontal Wells in Ultra-Low Permeability Reservoirs: A Case Study in HQ Oilfield, Ordos Basin, China

Abstract

The ultra-low permeability oil reservoir in the HQ oilfield within the Ordos Basin exemplifies a classic “three-low” oil reservoir characterized by low pressure, low permeability, and low abundance. Upon the commencement of oil well production, substantial initial production decline and inadequate pressure maintenance levels are encountered. Consequently, these wells commonly face persistent low production issues resulting from ineffective water injection. Currently, the available technical approaches for repeated stimulation of such wells to enhance production and efficiency are limited, thereby restricting the effective utilization of the remaining oil reserves. In response to this challenge, this article presents an innovative technology tailored for high-efficiency re-fracturing to improve oil recovery in ultra-low permeability reservoirs. Grounded in the theory of multiple flow couplings and volume development, this technology introduces a novel integrated process encompassing seepage field reconstruction, fracturing, and oil displacement. This comprehensive approach culminates in an integrated energy replenishment methodology applicable throughout the entire reservoir’s life cycle. The proposed technology materializes a novel development method for ultra-low permeability reservoirs, centered on the principles of “seepage field reconstruction, integration of fracturing and oil displacement, multi-mode energy replenishment, and all-round displacement”. This integrated strategy ensures the efficient development of ultra-low permeability reservoirs. The successful implementation of this technology in the HQ oilfield is a notable achievement. Notably, the daily oil production of directional wells and horizontal wells significantly increased from 0.5 tons and 1.8 tons to 2.4 tons and more than 10 tons, respectively. Furthermore, the estimated ultimate recovery rate witnessed a substantial improvement from 5.2% to 17.3%. These compelling results underscore the potential of this technology in revitalizing the production of low-yield wells in ultra-low permeability reservoirs.

1. Introduction

During the initial phase of ultra-low permeability reservoir development, a seven-point well network was employed, utilizing a “water injection + fracturing” model [1,2,3]. However, this approach faced several challenges due to factors such as tight reservoir lithology and non-Darcy flow seepage [4]. Consequently, a significant initial decline in oil well productivity, inadequate maintenance of reservoir pressure, and widespread occurrence of low-production wells were encountered [5,6,7]. Presently, international attention in horizontal well development primarily focuses on fracturing methodologies such as hole patching and generalized repetitive fracturing [8,9,10,11]. Conversely, limited discussion is available on the application of water injection well network development for recurrent transformation. In contrast, China has predominantly implemented volume fracturing techniques, primarily targeting unconventional reservoirs, such as tight oil, characterized by natural microfractures [7,12,13,14]. However, the prevailing strategy post-transformation revolves around energy depletion development. Notably, there is a dearth of research regarding the re-fracturing of low-producing wells within ultra-low permeability water injection development reservoirs. Initial treatment methodologies aimed to enhance the fracture penetration ratio of individual wells subjected to re-fracturing in their early stages [15,16]. However, this approach proves ineffective when applied at the operational scale of matured oilfields, primarily due to limitations arising from reservoir lithology densification and in situ stress conditions [17]. Furthermore, an insufficient understanding of ultra-low permeability oil reservoir behavior post-fracturing hinders the achievement of anticipated production levels.

With the aim of achieving efficient development, this article thoroughly integrates the geological characteristics of the reservoir and the current development status. Through this comprehensive analysis, it elucidates the limitations of existing development technologies in effectively controlling this type of oil reservoir. Employing geological engineering integrated numerical simulation technology, a “geology-stress-fracture-matrix” coupling model is established. The “Geology-stress-fracture-matrix” model enables quantitative characterization of the distribution patterns of the pressure field, stress field, and seepage field before and after the re-fracturing of horizontal wells. Consequently, it provides theoretical guidance for the efficient development of re-fracturing horizontal wells in injection production well patterns in ultra-low permeability reservoirs. For old wells developed by area water injection, a novel technical concept is proposed, centered on “volume development, flow field reconstruction, fracturing oil displacement, and comprehensive energy replenishment”. This concept encompasses a comprehensive fracture control design method for remaining untapped reserves, an integrated process technology for flow field reconstruction, fracturing, and oil flooding, an integrated energy supplement method involving “energy supplement before fracturing, energy augmentation during fracturing, and energy supplement after fracturing”, and horizontal well fractures. The transformation of development methods and transformation model is synergized with intermittent injection mining and energy replenishment technology. As a result of these advancements, the annual oil production scale of the oilfield has remarkably increased from 36,000 tons to 205,000 tons. The cumulative oil increment has surpassed 300,000 tons, and the predicted recovery rate has significantly improved from 5.2% to 17.3%. Notably, this achievement has led to the establishment of PetroChina’s (Xi’an, China) first 200,000-ton tight oil reservoir transformation development model demonstration area. These outcomes provide effective technical means for the efficient development of this type of oil reservoir.

2. Reservoir Development Background

2.1. Geological Setting

The HQ oilfield is located at a burial depth ranging from 2000 to 3000 m, featuring an effective reservoir thickness of 10 to 30 m. The porosity levels vary between 11% and 12%, while permeability measurements fall within the range of 0.3 to 0.4 mD. The initial reservoir pressure is estimated to be between 16 and 19 MPa. Notably, the median radius of the channels is extremely small, consisting of nanoscale pore throats, which presents significant challenges for reservoir development. Furthermore, the reservoir exhibits pronounced heterogeneity, comprising multiple layers with a permeability variation coefficient of 0.65 [18,19]. Regional investigations and coring analyses indicate the presence of well-developed microfractures within the reservoir, with a density ranging from 0.76 to 1.08 fractures/m.

2.2. Current State of Oil Reservoir Development

The HQ oilfield is currently in the “double low” stage, characterized by a low oil production rate of 0.2% and a dynamic recovery rate of only 5% to 8%. This represents a significant deviation of 10 to 13 percentage points from the calibrated recovery rate. The development of effective displacement in water injection development is hindered by reservoir physical properties and poor sand body connectivity in certain reservoirs. Consequently, the current formation pressure maintenance level stands at a mere 84.9%, while the water injection effective rate is only 34.5%. Post water injection development, water breakthroughs in oil wells are a common occurrence, with a water breakthrough rate of 38.9%. The average water breakthrough period is approximately 24 months, and the multi-directional fracture-type water breakthrough characteristics are evident. Furthermore, the poor physical properties of the reservoir and the presence of natural fractures contribute to a high proportion of low-yield wells and long-stop wells in the main oil reservoirs. Currently, these wells account for 43.6% of the total well count, while their production ratio is a mere 6.1%. Notably, the issue of low production in horizontal wells is particularly pronounced. Conventional injection and production adjustments have demonstrated limited effectiveness and short validity periods, severely impacting the overall development level of the oilfield.

To address the challenges encountered in the development of ultra-low permeability reservoirs, existing development technologies primarily rely on precise injection and production techniques, complemented by periodic water injection adjustments. However, the effectiveness of these measures remains limited, with a success rate of only 25.6%. Profile control and flooding have shown effectiveness in over 1000 wells (28.6% effective rate). Despite these efforts, single-well production remains low, averaging only 0.87 tons per day. Notably, 67.7% of wells produce less than 1 ton of oil per day, and the initial production decline rate exceeds 25%. The limited adaptability of these techniques hinders effective reservoir management and overall development outcomes.

3. Methodology

In this paper, a comprehensive coupled “geology-stress-fracture-matrix” coupling model is constructed using an integrated geo-engineering numerical simulation method. This model enables quantitative characterization of the distribution patterns of the pressure field, stress field, and seepage field before and after re-fracturing. The insights gained from this model provide valuable theoretical guidance for the efficient re-fracturing of ultra-low permeability reservoirs.

3.1. Pressure Field Variation

Employing reservoir numerical simulation, we simulated reservoir pressure changes. The simulation results revealed that after long-term water injection and production in the wells, the pressure retention level is significantly low, reaching approximately 75% of the original formation pressure. This effect is observed beyond the near-well zone (Figure 1a). However, pre-fracturing energy replenishment effectively restores the pressure in a larger region of the near-well zone, bringing it back to the original formation pressure level (Figure 1b). Furthermore, the integration of fracturing flooding enables the overall formation pressure to be maintained at a higher level, reaching up to 120% of the original formation pressure (Figure 1c). This facilitates the formation of intricate fractures and plays a stabilizing role in the later stages of development, thereby promoting sustained production.

3.2. Stress Field Variation

In this study, a fully fluid–solid coupling calculation method is used to simulate the dynamic changes of the reservoir stress field. The results revealed that after prolonged water injection and oil production, the horizontal bi-directional stress in the formation was low, and the two-way stress difference was significant. Under these conditions, fracturing tended to create a single fracture (Figure 2a). However, pre-fracturing energy replenishment led to an increase in stress in both directions, reducing the horizontal two-way stress difference from 7.3 to 6.0 MPa (Figure 2b). Notably, after the integration of fracturing flooding, the two-way stress values exhibited a relatively large degree of increase, while the two-way stress difference value decreased. This change facilitated the formation of relatively complex fractures, thereby increasing the oil drainage area (Figure 2c).

3.3. Seepage Field Variation

A matrix–fracture flowline model is used to characterize the reservoir seepage field quantitatively. After prolonged water injection and production, the seepage pattern was observed to be primarily from the injection well to the extraction well (Figure 3a). However, pre-fracturing energy replenishment resulted in linear flow near the wellbore and a shortened seepage distance (Figure 3b). Furthermore, after the integration of fracturing flooding, the flow from the fracture to the wellbore became close to linear flow, achieving the shortest effective seepage distance between wells/fractures. This significantly increased the reservoir pressure and drainage volume, expanding the seepage range by 3–4 times. Consequently, the seepage volume increased from 85.7 × 10⁴ m³ to 282.85 × 10⁴ m³ (Figure 3c).

The aforementioned “three fields” simulation study reveals that ultra-low permeability oilfields are influenced by reservoir physical properties and connectivity conditions. Seepage primarily occurs within the matrix, resulting in the displacement of output from injection wells to production wells. Consequently, injection and production development pressure conduction are slow, and reserves are developed only in the near-well zone. However, by implementing volume fracturing combined with water injection huff and puff, the fluid can achieve efficient flow in the fracture–matrix dual medium under the influence of horizontal wells and volume fracturing. This enables production from horizontal wells relying on elastic energy. As a result, a single well can essentially fully exploit the reserves within the fracture control range.

The variations in fracture-controlled reserves associated with different fracture networks after initial fracturing and volumetric recompression of horizontal wells are analyzed by numerical simulation methods [20]. The results demonstrated that the initial fractured wells controlled limited reserves. However, after volume recompression, the coverage of fractures to interwell and interfracture reserves increased significantly. Consequently, the single-well fracture-controlled reserves were enhanced from 36,000 to 172,000 tons (Figure 4).

The estimated ultimate recovery (EUR) after the volumetric fracturing of 11 horizontal wells in the Yuan 284 block was calculated by combining the methods of decline curve analysis and model fitting (

Table 1. EUR prediction results before and after high-efficient fracturing of 11 horizontal wells in the Yuan 284 (10⁴ t).

Type	Well	Starting EUR	Diminishing Method EUR before High-Efficient Re-Fracturing	Numerical Simulation Method EUR after High-Efficient Re-Fracturing	Average EUR Value after High-Efficient Re-Fracturing	Increment of EUR
Pilot test well	QP49	2	2.9	3.1	3	1
	QP50	1.5	2.3	3	2.65	1.15
	QP47	1.7	2.1	2.7	2.4	0.7
Industrial test well	QP1	1.2	1.5	2.5	2	0.8
	QP14	1.2	1.6	2.7	2.15	0.95
	CP52-16	2.6	3.6	3.9	3.75	1.15
	CP50-14	1.9	3.3	2.7	3	1.1
	CP52-15	2.7	4.1	3.9	4	1.3
	CP50-13	2.1	3	3.7	3.35	1.25
	CP50-12	2.1	4.1	3.5	3.8	1.7
	CP50-15	1.6	5.7	4.3	5	3.4
Average value		1.87	3.11	3.27	3.19	1.32

). The analysis revealed that the average single-well EUR increased by 13,200 tons, from 18,700 to 31,900 tons. Furthermore, cross-plot analyses of the fracture-controlled reserves (Figure 5) and the EUR for the 11 horizontal wells indicated a positive correlation, with a goodness-of-fit of 0.77 (Figure 6).

The recovery rates using different extraction methods were predicted by combining the reservoir engineering method (Arps decline curve analysis method) and numerical simulation (

Table 2. Comparison of different methods for predicting oil recovery rates.

Methods of Prediction	Recovery Rate of Original Plan	Enhanced Oil Recovery of Prediction			Recovery Rate after High-Efficient Re-Fracturing
		Re-Fracturing	Inter Fracture Displacement	Total Increase in Oil Recovery Rate
Numerical simulation method	5.3%	8.0%	4.2%	12.2%	17.5%
Reservoir engineering method	5.2%	7.9%	4.2%	12.1%	17.3%

). The reservoir engineering method indicated a 7.9% increase in recovery after re-fracturing and a 4.2% increase after interfracture flooding, totaling 12.1%, compared to water injection extraction. In contrast, the recovery after high-efficient re-fracturing reached 17.5%. Similarly, the numerical simulation method showed that compared to water injection extraction, the recovery after re-fracturing increased by 8.0% and interfracture flooding by 4.2%, for a total increase of 12.2%. Once again, the recovery after high-efficient re-fracturing reached 17.3%. The reservoir engineering method and numerical simulation method yielded highly consistent predictions of the recovery rate, validating the accuracy of the prediction results. Moreover, the findings underscore the significant potential of high-efficient re-fracturing in enhancing recovery rates.

4. Integrated Development Model of High-Efficiency Re-Fracturing Flooding

In this study, a fracture control design method for remaining unexploited reserves, a high-efficiency re-fracturing flooding integration process, and an integrated energy replenishment strategy involving energy replenishment before fracturing, energy increase during fracturing, and energy replenishment by interfracture injection and production after fracturing are established. These techniques constitute a comprehensive approach for horizontal wells. A concept of integrated high-efficiency re-fracturing flooding, with the core principle of “volumetric development, flow field reconstruction, fracturing flooding, and comprehensive energy replenishment” is preliminarily formed. Compared to the conventional development mode, the new integrated high-efficiency re-fracturing flooding development model offers four key improvements: (a) advancement from traditional planar water drive to “energized fracturing + inter-well and inter-fracture flooding + seepage”; (b) transition from single matrix/fracture seepage to three-dimensional seepage in complex fracture network systems; (c) evolution from fixed well types (oil and water wells) to no well types (oil well fracturing/water well diversion) for integrated recharge; and (d) shift from a conventional water medium to a composite medium of flooding and seepage. The high-efficiency re-fracturing and oil flooding integration development model has substantially enhanced the development effectiveness of ultra-low permeability reservoirs and possesses the potential for widespread adoption and application. This model provides a technical foundation for further large-scale implementation.

4.1. Fracture-Controlled Design Method for the Remaining Unmobilized Reserves

The remaining untapped reserves are primarily distributed between interwells, interfractures, and old fractures with insufficient modification [21]. To maximize the fracture-controlled reserves, geo-engineering integration is used to optimize the sweet spot, and then the reservoir is subdivided and cut. The evaluation model of the geological sweet spot of the reservoir was established using the physical parameters obtained from well logging data. The eighth track represents the classification result of the geological sweet spot (Figure 7). Similarly, the evaluation model of the engineering sweet spot of the reservoir was established using the elasticity parameters obtained from well logging data. The thirteenth track represents the classification result of the engineering sweet spot (Figure 7). By integrating the results of the geological sweet spot and engineering sweet spot, a comprehensive evaluation of the reservoir is conducted. The fourteenth track represents the classification result of the comprehensive classification (Figure 7). Based on the comprehensive evaluation of the sweet spot of geo-engineering integration, re-fracturing is performed on the old fractures with insufficient initial modification, and densification of the fractures is carried out between the old modification segments.

Among these methods, the re-fracturing of old fractures with insufficient initial modification requires an evaluation of the old fracture’s effectiveness. To address this, a “4-step method” is established to evaluate the effectiveness of old fractures in horizontal wells (Figure 8). The specific process is as follows: observe whether the fracture pressure is evident from the initial fracturing curve; check whether the downhole packer is damaged; test whether there is a discernible signal from the downhole microseismic test; and inject fracturing fluid at a small displacement before repeating the fracturing and observe whether the formation is noticeably supplied with fluids. When it is confirmed that the formation’s fracture pressure is not apparent, the packer is undamaged, the microseismic test does not exhibit a clear signal, and the formation does not have a significant fluid intake, which indicates that the old fracture modification is insufficient.

4.2. High-Efficiency Re-Fracturing Flooding Integration Process

To overcome the challenges associated with the double-seal single-card process tubular column, such as outward movement, limited construction parameters, and inability to address subdivided cutting issues [22,23,24], a high-efficiency re-fracturing flooding integrated process tubular column specifically designed for horizontal wells (Figure 9) is developed and implemented. In existing horizontal wells, the innovative application of 4 ¹/₂″ small-set cemented wellbore reconstruction and subdivided cut volume fracturing technology has led to the establishment of a highly efficient re-fracturing flooding integration technology for horizontal wells, with “volume transformation, comprehensive energy supplementation, and synergistic seepage and drive” as the core principles. This approach has successfully maximized reserves in horizontal segment subdivided cutting and complex fracture control while meeting the design requirements of the oil production index after volumetric fracturing.

For old wells with casing damage after a production period, the casing of the old wellbore that has undergone initial shot hole fracturing is often incomplete, hindering efficient re-fracturing. To address this issue, it is necessary to reconstruct a new wellbore through small casing cementing, also known as small casing wellbore recreation technology, before implementing the integrated process technology of high-efficiency re-fracturing and flooding. This technology involves the following steps: high-strength gel material is added into the old wellbore to seal the old fracture and reduce leakage (Figure 10a); a small casing is lowered into the wellbore, and the old and new casings are secured with annular sealing material, ensuring the stability of the small casing (Figure 10b); and after constructing the new wellbore, high-efficiency re-fracturing is performed using the small casing. A dissolvable bridge plug is employed to drive out the oil (Figure 10c).

High-efficiency re-fracturing flooding integration technology encompasses flow field reconstruction, fracturing operations, and crude oil replacement [25]. Flow field reconstruction begins with a thorough recharge of the pre-pressurized zone, involving the injection of substantial volumes of oil-repellent fracturing fluids. In a single horizontal well, this may exceed 10,000 m³, while directional oil wells typically require between 2000 and 3000 m³. The primary objective of this injection is to restore formation pressure to over 95% of its original value. Subsequent volumetric fracturing of oil and water wells within the area effectively maintains formation pressures at levels exceeding 120% of post-fracture values, thereby significantly restoring formation energy. During the volumetric fracturing process, old fractures are predominantly utilized for repeated fracturing, while new fractures are densely distributed between the existing ones. As a result, the fracture density has increased from 1.2 segments/100 m to over 2.9 segments/100 m, with more than 60% of these fractures being newly formed. Furthermore, there has been a substantial enhancement in various operational parameters associated with the fracturing process. The feed strength has increased significantly from 2.6 m³ to 40.5 m³, the sand strength has improved from 0.58 t/m to 3.8 t/m, and the discharge rate has risen from 3 m³/min to 8–12 m³/min. These improvements collectively indicate a remarkable optimization of the fracturing process. In addition to these general enhancements, fracturing operations also involve the selective application of specialized techniques, such as water plugging and fine-layered fracturing, based on the specific conditions of each well. This strategic utilization of diverse technologies reflects a synergistic approach, capitalizing on their combined strengths to achieve an optimized fracturing process.

In addition, the newly developed fracturing and oil replacement agents, represented by CQH-1 (Figure 11), CQH-2, and CQH-3, are notable for their low interfacial tension properties and have been extensively utilized throughout the entire process [26]. Specifically, the interfacial tension of the novel fracturing and oil repellent agent CQH-1 is 10⁻³ mN/m. Furthermore, it exhibits an oil permeability replacement efficiency of 82.7%. As a result, the oil sighting cycle of the test wells was significantly reduced from 8.2 days to 3.0 days after the wells were put into production (Figure 12). After fracturing, the well is shut in for approximately 40–60 days to ensure permeability equilibrium before production is resumed.

4.3. Integrated Energy Replenishment Strategy

With the continued production of the old wells, the relatively rapid depletion of energy in the formation resulted in increased difficulty in stabilizing production, even though some measures were adopted [27]. To address this challenge, this study presents an innovative integrated energy replenishment strategy that encompasses pre-fracturing energy supplementation, fracturing energy enhancement, and post-fracturing energy supplementation. During the stage of pre-pressurization energy replenishment, 8000–10,000 m³ of fluid is injected into the horizontal wells, while 2000–3000 m³ is replenished into the directional wells. This intervention aims to restore formation pressure to over 95% of its original level. Subsequently, in the energy increase phase, rapid injection of fracturing fluid and high-pressure fluid elevates the pressure within the fracture–matrix system, thereby increasing the formation pressure to exceed 120% of its initial value. To illustrate the effectiveness of this approach, the CP52-11 horizontal well needs to be considered. When the cumulative volume of fracturing fluid reaches 8000 m³ during pre-fracturing energy supplementation, the formation pressure returns to 95% of its original level. Furthermore, upon reaching 20,000 m³ in the fracturing energy enhancement stage, the formation pressure rises to 121% (Figure 13). The final stage involves post-fracturing energy supplementation, during which formation pressure redistributes, and oil and water undergo complete exchange. For instance, the well QP22, with a horizontal section length of 744 m, has a cumulative replenishment of 10,000 m³ of energy before repetitive fracturing, and then the formation pressure level rises from 63.2% to 103.8% (Figure 14). After repeated fracturing, the well is shut in for 21 days, the pressure dissipates fully, the formation pressure redistributes, and the oil and water are replaced fully. After an additional 21 days of shut in, the seepage absorption reaches equilibrium, and the pressure change is no longer noticeable.

4.4. Interfracture Energy Replenishment Technology after High-Efficient Re-Fracturing

The process of segmented water injection and oil extraction in horizontal well fractures necessitates interfracture and interwell energy replenishment and displacement. This study employs the independently developed Y445 + Y341 drillable packer combination, coupled with wave code communication digital selective injection technology, to establish a pressure wave code communication intelligent injection and production process (Figure 15). This technology fulfills the criteria for segmented underground automatic measurement and adjustment, as well as surface control. The entire process string can withstand pressures exceeding 50 MPa, offering a technical direction for efficient and stable production after repeated fracturing.

Compared with conventional oil production technology, the series of technologies proposed in this study have three major improvements. (1) In terms of development concepts, in view of the problem that water flooding is difficult to displace in ultra-low permeability reservoirs, the traditional water flooding development concept is abandoned, the oil leakage channels are increased by repeated fracturing, and all wells are used as oil production wells. (2) In terms of the optimization method of the repeated fracturing scheme, the traditional method judges the insufficiently modified segments of the first fracturing curve, and the sweet spot area of the unmodified segments is judged by the logging curve as the optional segments for repeated fracturing. This study proposes a “four-step method” to evaluate the degree of modification of the initial fracturing, comprehensively evaluate the geological sweet spot and engineering sweet spot of the reservoir through logging data, and comprehensively select the unmodified area of the reservoir as the modification object of repeated fracturing. The new method reduces the interference of artificial judgment, improves the scientificity of segment selection, and greatly improves the degree of available remaining oil. (3) In terms of repeated fracturing technology, the fracturing displacement, liquid volume, and ground liquid volume of conventional technology are relatively small, and fracturing is immediately put into production; conventional fracturing fluid does not have the function of displacing and replacing crude oil. The technology proposed in this study has four advantages: it replenishes the energy of the formation in three ways so as to achieve more complex fractures through the imbibition and displacement of crude oil and uplift the formation pressure to increase more oil flow channels. The dual-seal single-card repeated fracturing technology breaks through the limitation of the traditional dual-seal single-card process and improves the standard of various fracturing parameters. For wells with incomplete casing, wellbore reconstruction is carried out to ensure the smooth progress of large-scale fracturing. The application of a new oil displacement agent, CQH-1, improves the displacement efficiency of crude oil.

5. Field Application

The innovative integrated high-efficiency re-fracturing flooding development technology was implemented in the ultra-low permeability reservoirs of the HQ oilfield. As a result, A total of 53 horizontal wells were constructed in the Yuan 284 area from 2020 to 2023, and the daily oil production of the main horizontal wells was distributed between 7.5 and 13.3 tons, with an average of 10.2 tons (Figure 16). Similarly, the average daily oil production for the 36 directional wells rose from 0.5 to 2.4 tons, exhibiting a gradual improvement in the trial effect. Moreover, the average daily oil production for the 54 water injection wells reached 1.6 tons, accompanied by an 8% reduction in the water ratio compared to the pilot test. Adhering to the principles of comprehensive energy supplementation and continuous transformation, a total of 140 wells were commissioned in the industrial trial area. This intervention led to a remarkable increase in daily oil production from 105 to 562 tons. Consequently, the oil recovery speed accelerated from 0.30 to 1.19%, resulting in an annual oil production scale expansion from 36,000 to 205,000 tons. Notably, the cumulative increase in oil exceeded 300,000 tons, while the predicted oil recovery rate rose from 5.2% to 17.3%. The successful application of this technology has propelled the HQ oilfield to become the first ultra-low permeability reservoir within PetroChina to showcase efficient repetitive fracturing at the 200,000-ton level.

Compared with the existing oil production methods. In terms of efficiency, two main transformation technologies have been optimized and upgraded. The first is to reduce the sliding wear of the packer by optimizing the anchor claws of the water anchor, improve the pressure-bearing capacity by improving the material of the packer rubber sleeve, increase the single-trip tool operation capacity from 2.3 to 4.0 sections, and increase the sand production from 325 to 518 m³, and the fracturing cycle of a single well is reduced from 45 to 36 days. The second is to establish a standard operation card for wellbore reconstruction based on the previous test and continuously improve the speed of casing landing, cementing, and other links. The construction period of wellbore reconstruction has been shortened from the previous 32 days to 15 days. In terms of cost effectiveness, the application of horizontal well “four-step method” original fracture effectiveness evaluation process and reservoir engineering sweet spot evaluation model can realize intelligent optimization and auxiliary decision making of the repeated fracturing sweet spot, and the casing external loss rate is reduced from 16.7% to 8.12%. By optimizing the design of the scheme, the fracture design is further accurate, and the single-well cost is reduced from 15.27 million/well to 13.79 million/well; under the condition of fixed oil price (USD 45/barrel), the output–input ratio of horizontal wells in the 15-year evaluation period from 2019 to 2023 is 1.16–1.64, with an average value of 1.39, and the internal rate of return is 8.5–20.3%, with an average of 12.2%. Under the condition of stepped oil price, the output–input ratio of horizontal wells in the 15-year evaluation period from 2019 to 2023 is 1.39–1.96, with an average value of 1.67, and the internal rate of return is 10.8–23.5%, with an average of 16.3%. In terms of environmental impact, since it is difficult to establish a displacement system when water injection is developed in ultra-low permeability oil reservoirs, this technology converts water injection wells into oil production wells, changing from supplementing formation energy by water injection in the past to supplementing formation energy by injecting small-volume oil displacement fracturing fluid before fracturing, injecting large-volume oil displacement fracturing fluid during fracturing, and shutting in wells after fracturing. It greatly reduces the ineffective water injection project and at the same time increases crude oil production, which effectively alleviates the water shortage problem in Longdong and northern Shaanxi.

We compared the improvements of the new oil recovery method in terms of efficiency, cost effectiveness, and environmental impact compared with the traditional oil recovery technology. (1) In recent years, the research and development test of chemical plugging wellbore reconstruction and pre-plugging technology has achieved certain results, but it still faces the problems of low water cut reduction effect and long liquid production cycle. This study solves the problems of a low water cut and a long liquid production cycle by strengthening well selection, giving priority to wells with reliable geological reserves, and sealing water-bearing fractures by wellbore reconstruction. (2) For horizontal wells with poor reservoir physical properties and low regional formation energy at the edge of the oilfield, the initial production reaches the standard after volume fracturing, but the decline is relatively fast, and only low-level stable production can be maintained. This study optimizes the pre-pressure replenishment fluid volume according to the pressure test results of this well through regional continuous energy replenishment. For wells with rapid pressure drops after energy replenishment, multi-stage energy replenishment is carried out to achieve the purpose of slowing down the pressure drop, avoiding insufficient formation fluid supply after production, and increasing production. (3) During the energy replenishment or fracturing of horizontal wells, directional wells, and water injection wells, interwell fracture communication may occur due to the influence of natural fractures, which will affect the transformation effect. For the wells with fractures connecting or energy replenishment after fracturing in this study, under the premise of ensuring the well control safety of the adjacent wells, the dominant clusters are dynamically blocked to promote the extension of the unbalanced clusters, avoiding the continuous extension of the easily extendable fractures, which may cause a more serious connection with the adjacent wells, leading to an increase in the pressure of the adjacent wells, and resulting in the ineffective fracturing of the well. The unextendible fractures get fully extended to add new oil flow channels.

6. Conclusions

Ultra-low permeability reservoirs are characterized by unfavorable physical properties, high seepage resistance, and low reservoir pressure maintenance, resulting in numerous low-producing wells. Additionally, the pronounced heterogeneity of sand body structures poses challenges in establishing effective waterflooding displacement, rendering conventional waterflooding methods inadequate for efficiently mobilizing ultra-low permeability reservoirs. This study presents an innovative integrated high-efficiency re-fracturing flooding development technology to enhance the mobilization efficiency of ultra-low permeability reservoirs. Initially, numerical simulation results validate that the seepage field can be transformed from traditional water injection-driven displacement to fracture-controlled near-linear flow by implementing this technology. This transformation achieves the shortest effective seepage distance between wells and fractures, enabling comprehensive mobilization of reserves within the fracture-controlled zone. Subsequently, a novel integrated high-efficiency re-fracturing flooding development model is developed. Centered on the fundamental concepts of “volumetric development, flow field reconstruction, fracturing-driven oil extraction, and comprehensive energy supplementation”, this model optimizes the fracture network system through a fracture-controlled design method for remaining unmobilized reserves. It maximizes reserve control by employing subdivided cutting of horizontal sections and intricate fracture control in high-efficiency re-fracturing flooding processes. The integrated energy supplementation method applied before, during, and after fracturing extends stable production periods. Interfracture and interwell energy replenishment and displacement are facilitated by the technology of water injection and oil extraction between horizontal well fractures [28]. Finally, the new integrated high-efficiency re-fracturing flooding development technology was successfully deployed in the ultra-low permeability reservoirs of the HQ oilfield. Within the industrialized test area, the cumulative production of 140 wells significantly increased the recovery rate from 5.2% to 17.3%. Consequently, the novel integrated high-efficiency re-fracturing flooding development model proposed in this study effectively enhances the development of ultra-low permeability reservoirs and provides a valuable technical reference for the efficient exploitation of such reservoirs.