Effectiveness Of Hydraulic Fracturing Research Articles

Research progress in natural gas hydrate reservoir stimulation

Natural gas hydrate (hereinafter referred to as hydrate) is a kind of future energy with great potential. At present, the apparent factor hindering commercial hydrate development is the unqualified gas production rate, so it is necessary to research stimulation means from the perspective of hydrate exploitation method, and reservoir stimulation technology is particularly important. After systematically reviewing hydrate reservoir stimulation theories and technologies, this paper divides the field of reservoir stimulation into three directions, i.e., hydraulic fracturing, near-well stimulation and chemical stimulation, summarizes the main research results, problems and challenges in these three directions, and carries out discussion and prospect. And the following research results are obtained. First, rapid progress has been made in the research of hydraulic fracturing and it results have verified the fracability of hydrate deposits. It is revealed that the fracturing mechanism of hydrate deposits is similar to the tensile failure of rocks. It is indicated that fractures can still make contribution to permeability improvement after they closed under confining pressure, which is beneficial to secondary opening. In addition, the effects of in-situ stress and fracturing fluid on fracturing behaviors of hydrate deposits are analyzed, the numerical model of fracturing process is established, the mechanisms of hydraulic fractures to improve the depressurization and heat injection effects of hydrate reservoirs are analyzed, and the improvement effects of hydraulic fractures, as high-permeability channels on gas production rate and other key production indexes are clarified. Second, the research of near-well stimulation reveals the washout mechanisms and laws of high-pressure water jet on hydrate deposits. It is verified by numerical simulation that water jet slotting, circumferential grouting column and stratified split grouting can effectively improve the production performance, but the research is in the stage of establishing gas production model to evaluate the stimulation effect and the feasibility and effectiveness needs verifying. Third, as for chemical stimulation, the potential of CO2 caprock reconstruction and methanol huff and puff to improve the production performance of hydrate reservoirs is explored, and the feasibility of CO2 caprock reconstruction and storage is analyzed. In conclusion, to deal with current problems and challenges, it is necessary to strengthen the construction of the reservoir stimulation theory and technology system with hydraulic fracturing as the leading role, near-well reservoir stimulation as the auxiliary and chemical stimulation as the supplement, so as to accelerate the industrialization process of hydrate resources.

A practical analytical model for performance prediction in unconventional gas reservoir

Ninety percentage of newly proven natural gas reservoirs of China are mainly unconventional resources, which can be typically developed by multi-stage fracturing horizontal well technology. Two regions typically occur near each fracture after hydraulic fracturing, in which the lower permeability region is considered as the storage source and the higher permeability region as the flow channel. The existed analytical models so far are mainly derived by Laplace transforming. In this paper, an improved practical analytical solution is derived for unconventional gas reservoirs bypassing the Laplace transform and numerical inversion. Through solving material balance equation and adopting the integration, a rate vs. pseudo-time solution in real-time domain can be directly obtained. Five numerical cases are created to verify the accuracy of the proposed analytical solution and the ratio of regular/irregular region pore volume is also proved to be derived reversely by the output parameters, which is significant for the field engineers to evaluate the effect of hydraulic fracturing. Moreover, a field example of a multi-fractured horizontal well in a tight gas reservoir is provided for demonstrate application.

Quantitative Evaluation of the Competing Effects of Wastewater Disposal and Hydraulic Fracturing on Causing Induced Earthquakes: A Case Study of an M3.1 Earthquake Sequence in Western Canada

AbstractPrevious studies mainly attribute injection‐induced earthquakes (IIE) to hydraulic fracturing (HF) operations in the Western Canada Sedimentary Basin (WCSB), whereas the role of wastewater disposal (WD) has been largely overlooked. One particular reason is that HF operations usually exhibit a clearer spatiotemporal relationship to IIE than WD. However, ignoring the effects from WD when investigating the seismogenisis of IIE in the WCSB may not be appropriate, especially when both types of injection activities are present in the vicinity of the epicentral area. Here, we conduct a case study on an M3.1 earthquake sequence located in the WCSB that can be spatiotemporally correlated with both active HF and WD operations. We first build an enhanced catalog consisting of 256 events to delineate the relationship between the occurrence of IIE and injection history. We then investigate the source parameters of the mainshock in detail. Finally, we build a numerical model to calculate the Coulomb stress change caused by each type of injections on the two nodal planes of the derived focal mechanism. The result suggests that the M3.1 event probably occurred on a near‐horizontal nodal plane. In addition, the pore pressure diffusion from WD and the poroelastic stress transfer from HF could work collaboratively to cause an IIE. Therefore, stress perturbation caused by long‐term WD should also be considered in the seismogenic process of an IIE, especially when both active HF and WD are in the immediate vicinity of the IIE.

Fracturing criterion of rock hydrofracturing considering pore pressure effect

Traditional hydraulic fracturing theory believes that as the initial pore pressure increases, the breakdown pressure of the rock will decrease. Previous experimental studies have shown that the breakdown pressure of rock hydraulic fracturing may increase with the increase of initial pore pressure (gradient), which cannot be explained by traditional theory. The current understanding of the effect of pore pressure and its gradient during hydraulic fracturing is still unclear. In this study, the pore pressure effect of rock hydraulic fracturing is analyzed based on a large number of macroscopic and microscopic phenomena of rock hydraulic fracturing in the previous study. Then, a new fracture criterion of rock fracturing is built considering the pressure-gradient effect. This new fracture criterion can reflect the main influence factors, including the rock particle size, porosity, pumping flow, inner diameter of open hole section, length of main fracture, height of main fracture (or length of open hole section), fluid viscosity, pore pressure, minimum initial in situ stress and rock tensile strength. The new fracture criterion is examined by the rock fracturing experiment which considering the pressure-gradient effect. The results show that the proposed fracture criterion considering the pore pressure effect can well predict the breakdown pressure of rock, and the prediction trend is consistent with the experimental results. The average error is less than 1% when adopting the present fracture criterion. The parameter sensitivity of the fracture criterion is analyzed. Results show that the fracture pressure increases with the rock porosity and this trend becomes more apparent with a larger initial pore pressure. It shows that the fracture pressure increases with the pumped flow rate. Besides, it shows that the fracture pressure decreases when increasing the particle size of the rock, but the decreasing trend gradually slows down. The research results can provide a theoretical basis for the mechanism of rock hydraulic fracturing and the structural modification effect of fluids in rock engineering.

Investigating the Behavior of Permeable and Impermeable Reservoirs in Southern of Iraq

The effect of hydraulic fracturing on the permeability and fluid flow of various Iraqi oil storage cores has been studied. Rumaila, Majnoon, and Zubair fields with different permeability were included in this study. Experiments were performed to determine the breaking pressure of the permeable core. Therefore, numerical and experimental results were compared to test the constant pressure on the impermeable core. Finite Volume Method (EbFVM) corresponding to the Dual Porosity Dual Permeability Model (DPDP) was used. A fractured reservoir in the Iraqi fields was studied for its single-phase fluid flow behavior. To represent the pressure distributions, ANSYS-CFX program was used. The comparisons between the fractured and non-fractured cores has been presented. In this paper, the pressure for permeable cores of Rumaila field has been determined and verified during the experimental and numerical simulation due to its lower fracture pressure. Hence, the required pressure has been determined for impermeable cores of Majnoon and Zubair fields due to the difficulties to measure such high pressure. The results show that the expected pressure to make a flow for the core belongs to the Majnoon field reaches to 1.724×107 Pa (2500 psi), while the expected pressure for the fluid to flow into the core belongs to the Zubair field reaches 1.379×108 Pa (20000 psi).

In situ stress variation and coal reservoir permeability response of the Jurassic Yan'an formation in the southwestern Ordos basin, China: Its impact on coalbed methane development

The in situ stress (ISS) and permeability determine the coalbed methane (CBM) development method and extraction efficiency to a certain extent. Based on the data of the CBM well test and microseismic monitoring in the Jurassic Yan'an Formation, a prediction model of the ISS is established, and its influence on permeability and CBM development in the southwestern Ordos Basin, China, is discussed. The results show that the orientations of maximum principal stresses (Shmax) in the study area's eastern, central, and western parts are NE–SW, SEE–NWW, and NNW–SSE, respectively. With the increased burial depth, the main ISS parameters show an S-shaped variation. The study area's ISS state can be divided into Shmax > vertical stress (Sv) > horizontal minimum principal stress (Shmin) (<420 m), Sv > Shmax > Shmin (420 m–800 m), and Shmax ≈ Sv > Shmin (>800 m), and the transformation depth of the ISS field is 550 m and 1000 m. The lateral pressure coefficients (λ) ranged from 0.30 to 1.19, with an average of 0.69. The λ, Shmax/Shmin and Shmin/Sυ of the stress field are small, which indicates that the horizontal tectonic stress is weak. The coal reservoir's permeability is distributed between 0.0084 and 6.84 mD (av. 1.13 mD). Affected by the variation of the ISS state in the vertical direction, the coal reservoir permeability shows obvious zoning. The tensile stress field is beneficial to high-permeability coal reservoir formation, but it is constrained by the coupling constraints of a coal reservoir's confining pressure and effective stress. The coal reservoirs are in a normal fault stress state, which is beneficial for improving their hydraulic fracturing effect. Since the Shmin in the study area is always smaller than the Sυ, the height of the fracturing fracture should be controlled in the fracturing of the coal reservoir to avoid the overlying aquifer being connected by fracturing fractures. The above new insights have important guiding significance for the hyperpermeability CBM reservoir prediction, well-bore structure and hydraulic fracturing design in the Ordos Basin.

Laboratory study of the factors affecting hydraulic fracturing effect for inter-salt oil shale layers, Qianjiang Depression, China

This study aims to investigate the potential factors affecting hydraulic fracturing of inter-salt oil shale reservoirs in the Qianjiang Depression, China. Using the inter-salt shale samples, the re-crystallization seepage tests, rock mechanical tests under high temperature and pressure, salt rock creep tests, and direct shear tests were conducted. The testing results suggest several major factors that affect hydraulic fracturing effects in the end. First, the seepage of reservoir and fracturing fluid through hydraulic fractures leads to salt dissolution and crystallization, reducing the effective seepage area of fractures. Second, the salt crystal may block the pore throats or micro fractures after brine invades the shale, decreasing the overall permeability. Third, the low strength and obvious plasticity of inter-salt shale and the strong creep characteristics of salt rock raise difficulties for proppant to effectively support fracture walls, thereby sharply narrowing the hydraulic fracture width. Lastly, the weak interfaces (bedding planes and lithology interfaces) in inter-salt oil shale reservoirs restrict the height of hydraulic fractures, resulting in the disconnection of seepage channels between multiple inter-salt shale reservoirs. Thus, several factors together reduce reservoir permeability, weaken the fluid flow capacity in the fracture, narrow the fracture width, and limit the effective stimulation volume, resulting in weaken the effect hydraulic fracturing.

Crack Propagation during the Process of Carbon Dioxide Fracturing Based on Molecular Dynamics Simulations

Carbon dioxide (CO2) fracturing is a powerful strategy for enhancement of oil recovery by creating more fractures in the formation so that it can efficiently improve the flow conditions of oils and gases underground. In the present study, we explore the whole process of crack propagation by injecting CO2 molecules into the formation based on the molecular dynamics simulation. For the I-type crack of a rock sample, the details on the crack propagation are demonstrated, accompanied by the damage process near the crack tip. Next, the fracture toughness of the I-type crack is calculated according to the critical intensity factor and the energy balance equation, and the two results are in good agreement. Moreover, the CO2 fracturing process is probed when the rock includes natural fractures, and all of the cracks are finally merged after propagation. The effects of CO2 fracturing and hydraulic fracturing for breaking rocks are compared, and it is found that the first method is more conducive to the formation of multiple fractures and complicated crack surfaces. The reason is attributed to the expansion effect of the CO2 molecules, and the diffusion coefficient and the radial distribution function are calculated. These findings are beneficial to depict the details of rock fracturing and provide some inspirations on inventing effective techniques of oil and gas development.

A control method for hydraulic fracturing of the hard roof with long and short boreholes

During the mining of the working face, the hard roof can lead to a series of problems, such as air leakage in goafs, gas emission and concentration, casualties, and damages to buildings and ecological environment. The treatment of the hard roof has always been a difficult problem in major coal-producing countries. Although many scholars have conducted some theoretical research and field practice in this field in recent years, the aforementioned safety problems still occur frequently. In order to understand the mechanism of crack propagation caused by hydraulic fracturing of the roof, based on the theoretical analysis of linear elastic fracture mechanics and coal rock mechanics, the shear fracture criterion was determined for the formation of horizontal fractures in the natural cracks. The anisotropy of the crack tip, the adhesion forces in the coal and rock fracture surface and the conditions for hydraulic-induced crack propagation were also considered. The formation of cracks involves a vertical main crack and multiple extended-airfoil branch cracks which interconnect transversely and longitudinally. Under the effect of hydrodynamic water wetting and hydraulic fracturing, the stress of coal and mass changed. Meanwhile, the mechanical parameeters, such as compression, tensile strength and elasticity modulus and etc., were weakened, thus achieving the goal of structural transformation and slight weakening of coal and rock mass. According to the bidirectional stress of coal and rock mass, the maximum principal compressive stress of crack opening was calculated, and the propagation mechanism of hydraulic fracturing cracks in the hard roof was obtained. The hydraulic fracturing technology for controlling the hard roof was illustrated and has been applied in No. 2 Well of Sihe Coal Mine for many years. The research results could provide reference for the control of the hard roof in other similar mines.

Heterogeneous structural responses of high-rank coal pores to hydraulic fracturing based on low-field nuclear magnetic resonance

Pores in coal are not only the main space for coalbed methane (CBM) occurrence but also the space to be opened during CBM recovery. Therefore, the analysis of the impact of hydraulic fracturing on coal pores, especially the change of adsorption pores before and after hydraulic fracturing, is of great significance to the evaluation of the hydraulic fracturing effect and CBM recovery. Hydraulic fracturing experiments and low-field nuclear magnetic resonance technology were used to analyze changes in the T2 curve, adsorption pore, and the effects of distance and in situ stresses on pore modification in coal samples of the Sihe (SH) and the Chengzhuang (CZ) mines before and after hydraulic fracturing. The results show that hydraulic fracturing can affect pores &amp;lt;10 nm. The CZ coal samples exhibit stronger heterogeneity than the SH coal samples after hydraulic fracturing, and the pore size distribution (PSD) anisotropy of the CZ samples is increased. For the SH coal samples with poor heterogeneity, the effect of hydraulic fracturing on pore transformation depends more on in situ stresses. The smaller horizontal in situ stresses difference facilitates the establishment of complex pore networks. After hydraulic fracturing, when the pore diameter is 30–100 nm, the pore volume proportions of the SH samples in the directions of maximum horizontal principal stress (σH), minimum horizontal principal stress (σh), and vertical stress (σV) increase from 43.73% to 64.84%, 59.79%, and 60.16%, respectively. Hydraulic fracturing increases the anisotropy of the PSD of the CZ samples.

Study on Stability and Control of Pre-excavated Withdrawal Channel under Mining Influence

Abstract Pre-excavated withdrawal channel (PWC) is an effective means to shorten the withdrawal time of fully mechanized working face and improve the efficiency, safety, and reliability during the withdrawal. However, the key to the success of the withdrawal method is the influence of mining on the stability of PWC, taking the III32upper1 working face of Zhuzhuang Coal Mine as the engineering background. By theoretical analysis and numerical simulation, the deformation of the PWC is analyzed and the mechanical model of the influence of the instability of the main roof fracture on the PWC is established. And the effect of the fracture and rotation of the main roof on the PWC is analyzed. The result shows that the instability of the coal pillar leads to the fracture and rotary deformation of the main roof as the width of the coal pillar gradually decreases, which further aggravates the deformation of the PWC and the degree of ground pressure behavior. Based on the influence of mining on the PWC, a control method is proposed. This method uses hydraulic fracturing technology to weaken the mining stress and prevent the fracture of the main roof above the PWC. The control effect of hydraulic fracturing on PWC is analyzed through the establishment of numerical calculation model. The result of engineering practice demonstrates that the mining stress is significantly reduced, and the deformation of surrounding rock in the PWC is effectively controlled after hydraulic fracturing.

Techniques for improving fracture-controlled stimulated reservoir volume in ultra-deep fractured tight reservoirs: A case study of Kuqa piedmont clastic reservoirs, Tarim Basin, NW China

Based on analysis of the reasons for low efficiency and low production after fracturing of some wells in the ultra-deep fractured tight reservoirs of the Kuqa piedmont zone, Tarim Basin and the matching relationship between the in-situ stress field and natural fractures, technological methods for creating complex fracture networks are proposed. Through theoretical study and large-scale physical simulation experiments, the mechanical conditions for forming complex fracture network in the Kuqa piedmont ultra-deep reservoirs are determined. The effectiveness of temporary plugging and diversion, and multi-stage fracturing to activate natural fractures and consequently realize multi-stage diversion is verified. The coupling effect of hydraulic fractures and natural fractures activating each other and resulting in “fracture swarms” is observed. These insights provide theoretical support for improving fracture-controlled stimulated reservoir volume (FSRV) in ultra-deep tight reservoirs. In addition, following the concept of volume fracturing technology and based on the results of fracture conductivity experiments of different processes, fracturing technologies such as multi-stage fracture-network acid fracturing, “multi-stage temporary plugging + secondary fracturing”, fracturing of multiple small layers by vertically softness-and-hardness-oriented subdivision, and weighted-fluid refracturing are proposed to increase the FSRV. New environment-friendly weighted-fluid with low cost and new fracturing fluid system with low viscosity and high proppant-carrying capacity are also developed. These techniques have achieved remarkable results in field application.

Continuous three-dimensional stress monitoring in roof of coal mines for investigating the rockburst control effect with hydraulic fracturing

The occurrence of rockburst in coal mines is closely related to the stress in coal and rock mass. Through hydraulic fracturing measurements in roof, the three-dimensional stress in roof before and after hydraulic fracturing as well as during working face advancing was monitored, and the effect of hydraulic fracturing in roof on controlling rockburst was studied. The test results show that (1) after hydraulic fracturing, the three principal stresses in roof decreased remarkably, whose maximum reduction was about 20%, while the elastic strain energy in roof decreased by about 31% as well; (2) as the working face advanced, the three principal stresses in roof in front of the working face would increase continuously until reaching peaks and induce the strata fracture, and the variation of the elastic strain energy in roof was basically consistent with that of the magnitude of the stress; (3) with hydraulic fracturing, the position of the peak stress moved from 11 to 21 m in front of the working face, the peaks of the three principal stresses decreased, whose maximum reduction was about 32%, and the peak of the elastic strain energy in roof also significantly decreased by about 51%. The field investigation shows that hydraulic fracturing in roof can release the stress and elastic strain energy in coal and rock mass instantaneously, and will reduce the risk of rockburst; hydraulic fracturing in roof can be an effective pre-destressing method to reduce the stress and energy concentration in coal and rock mass during mining, and will prevent the risk of rockburst.

Mechanism and Numerical Simulation of Vertical Fracture Propagation in Composite Coal Rock

Vertical fracture propagation mechanism is important to understand the effect of hydraulic fracturing on the roof of outburst coal seams. In this paper, the differences in physical parameters and in situ stress between an outburst coal seam and its roof strata were compared, and influencing factors of roof-fracturing fractures connecting coal seams were vertically analyzed. The impact of different fracturing strata and injection rates on fracture propagation was studied by numerical models. Results show that the horizontal principal stress of an outburst coal seam is less than that of roof strata, and the fracture length of roof fracturing is larger than that of an outburst coal seam. Roof-fracturing fracture of an outburst coal seam has the material conditions to communicate downward with the coal seam. The downward propagation height of roof-fracturing fracture is positively correlated with the minimum horizontal stress difference between an outburst coal seam and its roof strata. Soft coal fracturing cannot form a long fracture dominated by tensile failure, so the coal seam can be communicated by transforming the roof strata of soft coal to form vertical fractures. The injection rate affects the failure location, fracture width, and fracture-propagation path of the numerical model. Construction parameters should be reasonably designed in accordance with the physical parameters of coal and rock, construction displacement, and in situ stress. These research results can provide a theoretical basis and data support for the design, and optimization of construction parameters of roof fracturing in outburst coal seams.

Optimization of long-term fracture conductivity with quartz: An experimental investigation

For the purpose of cost management, many companies and research institutions have carried out technical research on the replacement of ceramic with quartz sand as proppant. However, few studies have systematically carried out short-term and long-term conductivity evaluation experiments for quartz sand proppants with different particle sizes. It is urgent to conduct experiments on the conductivity of quartz sand, and optimize the fracturing design based on the experimental results. The conductivity is the key indicator of the fracturing effect. The greater the conductivity, the better the hydraulic fracturing effect, and the longer the effective period, the higher the fracturing benefit. In this study, by evaluating the long-term conductivity of proppant, the influence of the type of quartz sand proppant, proppant particle size and sand paving concentration on the fracture conductivity are studied. Main results show that 1) the proppant performance of quartz sand produced in Chifeng is better than that of produced in Tongliao; 2) The long-term conductivity experiment results show that the conductivity of quartz sand proppant produced in Chifeng is better than that of TL, and under the same sanding concentration and closing pressure, the larger the particle size of the quartz sand, the higher the conductivity, and the greater the decrease in conductivity with the increase of the closing pressure; 3) In the influence of sand paving concentration, when the sand paving concentration is 5 and 7.5 kg/m2, the conductivity increase with the increase of the sand paving concentration.

Propagation path tracing of hydraulically created fractures based on microseismic monitoring

To evaluate the effect of hydraulic fracturing in a low-permeability reservoir, a propagation path tracing method for hydraulically created fractures was established based on microseismic monitoring data. First, the numerical simulation of the wave propagation process, grid search, and error-weight coefficient method was combined to locate the microseismic source. Then, the moment tensor inversion method was used to determine the tensile angle and source mechanism of hydraulically created fractures. Next, the tensile angle was used as the weight-index to determine the size of the mixed-source mechanism fracture combined with the shear and tensile source size quantization model. Finally, the spatial topological relationship between fractures was determined by comprehensively considering the spatial location and radius of the fractures, to realize the propagation path tracing of hydraulically created fractures. These tracking results can be used as one of the bases for the evaluation of the hydraulic fracturing effect.

Experimental study on stress monitoring in fractured-vuggy carbonate reservoirs before and after fracturing

Natural fractures and caves serve as the main reservoirs of fractured-vuggy carbonate reservoirs, and acid fracturing is the main technical means to improve the well productivity. Only if hydraulic fractures connect the natural fractures and caves are there obvious hydraulic fracturing effects. However, the faults, caves, and natural fractures distributed in the bedrock resulted in the distribution of the complex in-situ stress field, which restricts the direction and geometry of hydraulic fracture, thus deciding whether hydraulic fracture could connect with natural fractures and caves. Therefore, an accurate description of the magnitude and direction of the stress field in different regions of faults and caves is of great significance. In this paper, a series of hydraulic fracturing experiments on artificial carbonate samples with presetting faults and caves were performed. And high-precision strain sensors prefabricated in the rock sample were used to monitor the stress field during fracturing. Four kinds of fault-cave geological combinations are investigated, and the results show that the presence of fault-karst can significantly change the distribution of in-situ stress and result in deflecting of the fracture, which may or may not be beneficial to connection of caves and faults, depending on well placement. There is high stress concentration at the end of the fault, and the maximum and minimum principal stresses are obviously higher than the far-field stresses. The lateral stress of a single fault is very low in the direction perpendicular to the fault plane, and there is little change in the direction parallel to the fault. Due to low maximum and minimum stress, the intersection of intersecting faults will exert a kind of “attraction” to hydraulic fractures. The stress distribution around the cave is lower than that in the distance. The results can provide theoretical guidance for optimizing hydraulic fracturing design in fractured-vuggy carbonate reservoirs.

The exploration and exploitation of shale gas in Algeria: Surveying key developments in the context of climate uncertainty

Algeria ratified the 2015 Paris Agreement on Climate Change which is committed to reduce greenhouse gas emissions through renewable energy promotion, expanding forest areas and improving water resources. However, the exploration and the exploitation of shale gas are authorized in Algeria. Here, we discuss the socioeconomic factors that have led Algeria to authorize shale gas exploitation regardless of the potential effects of hydraulic fracturing on biodiversity loss and human health under climate change uncertainty context. Data reported show the difficulty to understand the multifaceted aspect of shale gas impacts. Indeed, without a comprehensive environmental assessment (air, soil, water and biodiversity) and human health impacts under climate change context, there is no clear evidence regarding the real costs, on the one hand, and the palpable benefits, on the other, of shale gas exploitation. Scientific data actualization and fully recognizing evidence in the literature are recommended when evaluating the potential adverse effects of unconventional gas on human and animal health, and also when creating hydraulic fracturing legislation. The development of alternative ecofriendly tools and methods to fossil energy sources (e.g., solar photovoltaics, wind, and geothermal energy) has become an emergency to help diversify the economy and safeguard natural resources for future generations.

Determination of the degree of coal deformation and its effects on gas production in the southern Qinshui Basin, North China

Coal deformation significantly impacts coalbed methane (CBM) productivity. However, existing methods are incapable of quantitatively evaluating coal deformation on a high accuracy when there is limited core recovery from a CBM well. This study proposes a well logging coal structure index (WCSI) to quantify the degree of coal deformation. The WCSI of targets is calculated based on the data from well logs and coal cores, and it can also be estimated from well logs when no coal cores are recovered from wells. Thus, the WCSI can be applied to the regional-scale evaluation of coal deformation degree, and its effectiveness has been validated by data for 245 samples from the No. 3 coal seam in 55 exploration wells in the southern Qinshui Basin (SQB). Results indicate that the reservoir permeability and hydraulic fracturing effectiveness depend on the degree of coal seam deformation, which further affects the unique distribution of gas production in the known production regions in the SQB. Generally, the lowly deformed coal seam (WCSI (avg.) < 40) occurs mainly in the tectonically ‘stable’ regions (no faults or only small-scale faults development) where the micro-fractures/cleats in coal are not well developed. Low WCSI (avg.) commonly indicates low permeability (<0.06 mD) and poor hydraulic fracturing of the reservoir, resulting in relatively low gas production rate of wells in these regions. Secondly, the moderately deformed coal seam (40 < WCSI (avg.) < 60) is mainly found in areas close to moderate-scale faults, where coal seams have relatively high reservoir permeability (>0.06 mD) and hydraulic fracturing is easiest. This causes high gas production rate from wells in these regions. Thirdly, the highly deformed coal seam (WCSI (avg.) > 60) is generally found close to large-scale faults, indicating very low reservoir permeability (<0.03 mD) and the worst effectiveness from hydraulic fracturing of the reservoir, resulting in the lowest gas production rate of wells in these regions. In summary, the production efficiency from CBM wells in the SQB can thus be described in terms of the WCSI (avg.) value, and hence this parameter can help with preliminary forecasting gas production.

Possibility of using machine learning algorithms to assess the effectiveness of acid hydraulic fracturing

At the fields of the Perm Territory, there is a significant application of hydraulic fracturing technology to stimulate the production of carbonate reservoirs. The analysis of the efficiency of the acid hydraulic fracturing technology by the example of the fields of the Perm Territory is presented in the article. The effectiveness of the technology is largely determined by the geological and physical conditions of the fields. Using the methods of mathematical statistics, the main geological and physical parameters which most significantly affect the effectiveness of measures for acid hydraulic fracturing have been identified. A multiple regression model was built to find out the complex influence of the parameters on the increase in oil production after the measures. Various machine learning algorithms have been used to predict the increase in oil and fluid input rates after acid fracturing. Different algorithms showed different results. For the problems of forecasting oil production after the application of acid fracturing, the most successful one was the support vector method.