Low Permeability Gas Research Articles

Research and Application of an Environmental-Friendly Low-Damage and High Salt-Resistance Slick Water Fracturing Fluid System

The Sulige gas field is a typical low-pressure low-permeability tight sandstone gas reservoir. The reservoir has poor seepage capacity, strong heterogeneity, high mineralization of formation water and extremely scarce water resources on the site. These unfavorable factors have brought great difficulties to the on-site mining process. Now, a nano-composite green environmental protection slick water fracturing fluid system CQFR can be quickly dissolved because of the larger specific surface area, and the small molecular size makes the damage to the reservoir less than 5%, and the average drag reduction effect can reach more than 73%. It can quickly and well dissolve and maintain performance under high salinity conditions and fracturing flowback fluids. It responds well to the complex reservoir conditions on the construction site and makes the flowback fluid recyable, which greatly reduces the consumption of water resources on the construction site and effectively improves the construction efficiency and economic benefits.

Application of Heterogeneous Composite Model with Consideration of Stress Sensitive in Low Permeability Gas Reservoir

Stress sensitivity is a key factor affecting the productivity of single wells in low permeability gas reservoirs. A well test model for heterogeneous composite gas reservoirs under the influence of stress-sensitive effects was established. Based on the theoretical model, the well test was designed by gradually increasing the pressure difference. The relationship between abnormal high pressure and reservoir stress sensitivity was analyzed. Theoretical research shows that stress sensitivity will cause permeability damage during the production process, and the pressure drop test curve shows that the physical properties of the reservoir have gradually deteriorated. The pressure recovery test curve shows that the physical properties of the reservoir are getting better. Field practice shows that stress sensitivity is related to the formation of abnormally high pressure in the formation without considering the micro-cracks in the formation. Stress-sensitive reservoirs are generally unbalanced and compacted due to overpressure, for fluid expansion/conduction overpressure in Ledong Area. For these reservoirs, there is almost no stress sensitivity. The research results have significance for guiding the design and data interpretation of stress-sensitive reservoir.

Establishment of an initial water production evaluation chart for the gas province in the East China Sea

In order to reveal the reasons for the differences in initial water production characteristics of gas wells in different regions of the East China Sea Basin and to screen favorable targets for low-permeability reservoir reconstruction, a calculation method of initial production water–gas ratios at different gas column heights was established by combining gas-water two-phase capillary pressure with J function definition, power function relative permeability model and fractional flow equation. Then based on the experimental data of standard capillary pressure and standard relative permeability in the gas province of the East China Sea, an evaluation chart on initial production water–gas ratio of the East China Sea at different gas column heights was calculated and established. Finally, the evaluation chart was validated by using the data of the tested or commissioned gas wells in the gas reservoirs of the East China Sea, and the physical property conditions suitable for reservoir reconstruction of low-permeability gas reservoirs in the East China Sea were determined according to the evaluation chart. The following research results were obtained. First, the evaluation chart established in this paper reflects that the correlation between the parameters is similar to the actual production law and matches well with the actual data, which verifies the correctness and reliability of the chart establishment method and the key parameter calculation method. Second, with the production water–gas ratio less than 1 m3/104 m3 as the screening condition, for the permeability of the low-permeability gas reservoirs (with gas column height of about 50 m) in the west sub-sag, the central uplift belt and the periphery suitable for reservoir reconstruction shall be greater than 0.65 mD, and that of the large low-permeability gas reservoirs (with a gas column height of more than 100 m) in the north of central uplift belt shall be greater than 0.26 mD. Third, there is no gas–water contact in gas reservoirs under the hydrocarbon accumulation conditions of in-situ reservoirs, so the method and the evaluation chart proposed in this paper are not applicable. It is concluded that the method of establishing the initial water production evaluation chart and the method of calculating the key parameters proposed in this paper can provide meaningful reference for the development and evaluation of other gas fields in China.

Convection heat transfer and friction characteristics of liquid CO2-N2 foam fracturing fluid using C4F9OCH3 as frother

The liquid CO2–N2 foam fracturing fluid containing C4F9OCH3 causes no damage to formation, similar to dry CO2 fracturing, can be used to stimulate water sensitivity and low-permeability gas reservoirs. The main objective of this experimental study was to ascertain the convective heat transfer and frictional characteristics of this modified dry fracturing fluid system. A foam fracturing fluid test system was adopted in this study. The results showed that the viscosity of the liquid CO2–N2 foam fracturing fluid with C4F9OCH3 added was significantly higher compared with the liquid CO2–N2. The optimum frother concentration was 0.65%. The shear rate had the most significant effect on the convective heat transfer. The pressure had a significant effect on the frictional characteristics of the complex foam fluid due to the compressibility of the internal and external phases. When the flow velocity was greater than 1.4 m/s, the frictional resistance coefficients for foams of different qualities were similar. The velocities in the expression of the generalized Reynolds number, Re′, and the generalized Prandtl number, Pr′, were not sufficient to describe the influence of the shear rate on the heat transfer. When fitting the convection heat transfer coefficient, the effect of shear rate on coefficient of thermal conductivity was also considered. These results can be used to calculate the temperature field and predict the friction pressure drop in the use of this foam fracturing fluid.

Inversion of distributed temperature measurements to interpret the flow profile for a multistage fractured horizontal well in low-permeability gas reservoir

• Development of inversion model based on Levenberg–Marquart algorithm. • Interpreting flow profile for multistage fractured horizontal well (MFHW). • Interpretation of fracture/reservoir parameters from distributed temperature profile. • Simulation of temperature behavior of MFHW and orthogonal experiment design for sensitivity study. Distributed temperature sensors (DTSs) are gradually being used to monitor downhole conditions for multistage fractured horizontal wells (MFHWs) during production. However, significant challenges exist with respect to translating the downhole DTS data to flow profiles of MFHWs, especially for low-permeability gas reservoirs (LPGRs). This study aims to interpret the flow profiles for an MFHW in LPGR through the inversion of downhole distributed temperature measurements. Firstly, a comprehensive inversion procedure combined with forward and inversion models is developed. The forward temperature prediction model is used to simulate the temperature distributions of an MFHW in each inversion iteration. Based on the Levenberg–Marquart (L–M) algorithm, an inversion model is proposed to decrease the differences between the simulated temperature data and the observed temperature data. Subsequently, a sensitivity study using an orthogonal design table L 18 (3 7 ) is performed to determine the inversion parameters (fracture half-length and reservoir permeability). Then, an original case demonstrates the direct translation of the observed temperature to the flow profile. Finally, two more cases are presented to illustrate how to interpret the flow profiles based on the inversion results of the two aforementioned inversion parameters respectively. The satisfactory inversion results validate the accuracy and feasibility of the proposed inversion approach to predict flow profiles or diagnose fracture parameters from the DTS data of an MFHW in LPGR theoretically.

The application of modified isochronal well test in a low-permeability condensate gas field

ABSTRACTThe modified isochronal well test is a deliverability test method suitable for gas wells in low-permeability gas reservoirs. Generally, the absolute open flow of gas wells is determined by modified isochronal well test, thereby establishing a reasonable working system for gas wells. It is also possible to obtain gas layer physical parameters, gas reservoir engineering parameters, heterogeneity, boundary characteristics, etc. In this paper, the actual test data of a condensate gas field are applied. By modified isochronal well test analysis, not only the deliverability equation of the gas well is obtained, but also the absolute open flow is determined. At the same time, the formation parameters such as reservoir permeability and skin factor are obtained and the acidification effect is evaluated.

A New Method to Determine the Productivity Equation Coefficient of Low Permeability Gas Reservoir

Stress sensitivity exists in the development of low permeability gas reservoir, which leads to the large difference between the productivity of gas well in the middle-late stage and the initial stage. Gas field production usually uses empirical formula to calculate productivity equation coefficient by known well test data or static pressure. However, in actual production, due to economic reasons, well testing cannot be carried out frequently, or interrupted production to obtain the static pressure, so the actual productivity of gas Wells cannot be obtained quickly. This paper combined the material balance equation with binomial productivity equation, and the coefficient of productivity equation is obtained by using the fitting analysis of single well production history data. This method overcomes the dependence of traditional method on formation pressure, and the productivity equation which calculated by this method can reflect the influence of stress sensitivity on gas well productivity, it provides a new method for productivity analysis of low-permeability gas reservoirs.

Nuclear Magnetic Resonance Simulation Experiment for a Water Drive Gas Reservoir

The water invasion property and water drive gas displacement efficiency of water drive gas reservoirs are studied under different displacement pressure gradients by using nuclear magnetic resonance (NMR) online detection technology to better guide the scientific exploration of these reservoirs. The breakthrough pressures of the water seal and water lock are also analyzed. The results show that low-permeability gas reservoir water bodies pass through large pores preferentially and then pass through holes and small pores. The remaining gas is mainly distributed in holes and small pores. In contrast, high-permeability gas reservoir water bodies pass through large pores and holes preferentially, and the remaining gas is mainly distributed in large pores and small pores. As the permeability increases, the water drive gas displacement efficiency decreases. As the displacement pressure gradient increases, the displacement efficiency initially increases and then decreases. The breakthrough pressures of the water seal and water lock are highly affected by the permeability. Large permeability results in easy water breakthrough. Variations in the water invasion and water drive gas displacement efficiency are consistent with the variations of the breakthrough pressure and accurately reflect the properties of water drive gas reservoirs.

PRODUCTIVITY PREDICTION OF FRACTURED HORIZONTAL WELLS WITH LOW PERMEABILITY FLOW CHARACTERISTICS

Horizontal well and large-scale fracturing are revolutionary technologies in petroleum industry. The technologies bring obvious economic benefits to exploiting unconventional oil and gas reservoirs with low permeability, ultra-low permeability and shale gas. With the increasingly extensive application of these technologies, other correlated technologies have also gained great development. However, low-permeability reservoirs exhibit complicated features and horizontal well fractures have complex shape. The existing methods for the productivity prediction of fractured horizontal well in low-permeability reservoirs rarely consider the influencing factors in a comprehensive manner. In this paper, a horizontal well seepage model of casing fracturing completion was established according to the superposition principle of low-permeability reservoir and the relationship between potential and pressure, by which model the seepage characteristics of low-permeability reservoirs could be fully described. Based on the established new seepage model, a new targeted model with coupling seepage and wellbore flow was established for the productivity prediction of low-permeability fractured horizontal well. Finally, the new targeted model was verified through field experiment. The experimental results confirmed the reliability of productivity prediction by the proposed model. Sensitivity analysis was then performed on the parameters in the proposed model.

Transient-Flow Modeling of Vertical Fractured Wells with Multiple Hydraulic Fractures in Stress-Sensitive Gas Reservoirs

Massive hydraulic fracturing of vertical wells has been extensively employed in the development of low-permeability gas reservoirs. The existence of multiple hydraulic fractures along a vertical well makes the pressure profile around the vertical well complex. This paper studies the pressure dependence of permeability to develop a seepage model of vertical fractured wells with multiple hydraulic fractures. Both transformed pseudo-pressure and perturbation techniques have been employed to linearize the proposed model. The superposition principle and a hybrid analytical-numerical method were used to obtain the bottom-hole pseudo-pressure solution. Type curves for pseudo-pressure are presented and identified. The effects of the relevant parameters (such as dimensionless permeability modulus, fracture conductivity coefficient, hydraulic-fracture length, angle between the two adjacent hydraulic fractures, the difference of the hydraulic-fracture lengths, and hydraulic-fracture number) on the type curve and the error caused by neglecting the stress sensitivity are discussed in detail. The proposed work can enrich the understanding of the influence of the stress sensitivity on the performance of a vertical fractured well with multiple hydraulic fractures and can be used to more accurately interpret and forecast the transient pressure.

A new method to determine reasonable well pattern density in low-permeability gas reservoirs

The productivity evaluation of gas reservoirs is used to determine the optimal number of well pattern, which is an important part of the preparation of gas reservoir development plan. Using integral transformation and asymptotic analyses to solve the pseudo steady-state flow model in closed rectangular gas reservoir of constant thickness, analytic solution of pressure distribution of eccentric well can be obtained. This paper presents mathematical model of multiple wells producing from a closed gas reservoir by applying the principle of pressure superposition. The pseudo steady-state dimensionless production index is introduced to solve the multiple well mathematical models under the constant pressure, and the number of production wells and cumulative production in different production interval are obtained. The results of the calculation shows that the curve of the cumulative production and economic benefits have a maximum value when the producing time, gas price, and drilling costs are given, so it can determine the optimal number of wells and provide the basis for making gas reservoir development plan.

Investigating the pressure characteristics and production performance of liquid-loaded horizontal wells in unconventional gas reservoirs

The increasing exploiting activities in unconventional gas reservoirs renew the interests of investigating liquid loading phenomenon, a common production issue for gas wells. Most previous studies focused on predicting the onset of liquid loading. The reservoir pressure characteristics and wellbore production performance after the onset of liquid loading have not been systematically investigated. This work introduces a newly developed fully implicitly coupled wellbore reservoir model to investigate the liquid loading effects on horizontal wells in low permeability gas reservoirs. The model honors the mass and momentum balances in both reservoir and wellbore systems, and thus allows us to analyze both wellbore and reservoir dynamics after the onset of liquid loading. The simulation results reveal that horizontal gas wells in low permeability reservoirs might experience natural cyclical production after the onset of liquid loading, which coincides with field observations. Both uniform stimulation and hydraulic fracturing help mitigate the cyclical production phenomenon. The near-wellbore reservoir pressure buildup and wellbore fluid reinjection are also evaluated. This work demonstrates the successful application of the coupled wellbore reservoir model in predicting the rich production phenomena of stimulated horizontal wells after the onset of liquid loading.

Quantitative evaluation of low-permeability gas reservoirs based on an improved fuzzy-gray method

Accurate and quantitative evaluation of low-permeability reservoirs and other more complex reservoirs not only depends on various control factors related to the reservoir but is also affected by our incomplete understanding of reservoirs and the imperfections inherent in evaluation methods. The evaluation of reservoirs may be improved by determining the integrity and uncertainty of the reservoir evaluation process and effectively examining the differences between evaluation results. Taking membership as the linking factor, utilizing the idea of information superposition, and combining the fuzzy comprehensive and gray relational evaluation methodologies in the uncertainty evaluation, this study presents a differentiated and improved internal algorithm and establishes a more effective fuzzy-gray comprehensive evaluation method. The example of E32 interval evaluation in the southern Nanbaxian gas field of the Qaidam Basin shows that this method not only considers the fuzziness and grayness related to reservoir evaluation but also provides improved discrimination of low-permeability reservoirs and increased accuracy in the reservoir description. According to the improved reservoir classification results, the E32 intervals are divided into four types of reservoirs, of which types I and II are favorable reservoirs. The lateral distribution of the reservoir is described in more detail than previously, with near-continuous knowledge, and there is a good distinction between the reservoir type. The obtained results are in line with the development rules of geological deposition and the geological knowledge base, approximately matching existing practical results and geological knowledge. This method can be used to quickly and effectively guide exploration and development strategies related to gas fields, and provides a new approach for low permeability and tight reservoir evaluation.

A numerical well model considering capillary end effect for the low-permeability gas reservoir

In the recovery of low permeability reservoir, the capillary pressure has an important effect, which may reduce gas production. Due to the capillary end effect, there are two flow patterns in the vicinity of the production well: both water and gas phases can flow into the production well and only gas phase can enter the production well. Based on the analytical equations of one-dimensional radial flow considering the capillary end effect, an alternative numerical well model for low permeability gas reservoir is constructed. Numerical examples show that the proposed model can reflect the dramatic change for saturation and gas-phase pressure in the vicinity of the production well both for two flow patterns, and therefore can predict the gas and water productions accurately at different grid scales. In contrast, the original Peaceman well model for multi-phase flow, which is directly extended from that for single-phase flow, only can provide good prediction under the condition of that the grid size is enough small. Especially, for the original Peaceman well model, this problem is out of control since it is difficult to estimate a suitable grid size.

ГЕОЛОГО-ПРОМИСЛОВІ ОСОБЛИВОСТІ РОЗКРИТТЯ ТА ВИПРОБУВАННЯ СЛАБОПРОНИКНИХ ГАЗОНАСИЧЕНИХ ВІДКЛАДІВ

The relevance of research is attributed to the scientific justification of the features of disclosure and testing of low-permeable gas perspective deposits. Under the low-permeable gas perspective deposits in the Carpathian region of Western Ukraine, we consider the rocks of the Cretaceous system (based on previous works). Deposits of the Cretaceous system in the cross section of the Carpathian region occupy more than half the volume of the rock mass within the boundaries of the flysch strata, and the bulk in the depth interval 1.5–3.5 km corresponds to the sediments of the Stryiska suite of the upper section of the Cretaceous system. That is, the object of research is quite significant in its volume. In selecting the perforation intervals and methods of secondary action on the productive layer in wells drilled in regions where low-permeable thin-layered flysch formations (thin interlayering sandstones, siltstones and mudstones) are observed, we propose a method by which the choice of methods of secondary action on the layer, the intervals of mudstone is justified. In this case, the gas perspective deposits (due to the increase in the permeability of the mudstone layers during their perforation, and later also during the hydraulic fracturing of the layer) are combined into one significant in thickness gas-bearing exploitation object, which increases the flow of gas into the wells. Target disclosure of the gas horizons and increase of their gas output to optimal geological and economic parameters require the use of new technologies, similar to those used in the extraction of gas from shale rock. The simulation of the hydraulic fracturing process of the sand-mudstone pack of Upper Cretaceous deposits in the range 3260–3330 m in the well 2-Tarasivka with the Meer MFrac program proved an increase of the fixed crack opening (in this case, from 0.02 to 0.22 cm), which will contribute to significant increase in the conductivity of the crack from 668 to 6000 mD·m, and it is an important factor in the extraction of hydrocarbon gas from low-permeable deposits, which will increase the productivity of wells. And this will contribute to a significant increase in the fuel and energy supply of Ukraine by its own hydrocarbon raw material.

Gas Filtration in a Low-Permeability Formation with a Hydraulically Fractured Well

This paper presents the results of numerical calculation of gas filtration in a low-permeability gas formation with a fractured well. It is assumed that in the low-permeability formation and in the finite-conductivity fracture is described by the Darcy law. The Peng–Robinson equation was used to take into account the dependence of the PVT properties of the gas on pressure. We studied the effect of fracture conductivity and formation permeability on the distribution of pressure and gas properties in the “fracture–formation” system and on the dynamics of well pressure and gas production rate during gas well testing.

Fractal characteristics of low-permeability gas sandstones based on a new model for mercury intrusion porosimetry

Studying pore structure characteristics is essential for understanding and evaluating the storage and seepage properties of low-permeability reservoirs. Fractal geometry has been widely employed to study fluid flow in porous media. Although numerous fractal models have been proposed for determining the fractal dimensions from mercury intrusion curves, these models just considered the fractal dimension for pore space and did not consider the fractal characteristic of tortuous pore length, which has been demonstrated to exist in pore structures and been adopted by many investigators.Herein, a new model for calculating the fractal dimensions from mercury intrusion curves is developed by considering both the fractal dimension for pore space and tortuosity. The new model more accurately characterizes the pore geometries of fractal porous media. The sum of the fractal dimensions for pore space and tortuosity is obtained from the slope of log (SHg) vs. log (Pc), where SHg is the mercury saturation and Pc is the capillary pressure. The fractal characteristics and relations between the sum of fractal dimensions and pore structure parameters were examined. The results revealed that the inflection point divides the fractal curves into two segments with different slopes and thus two fractal dimensions were obtained at different pore size ranges. D1 reflects the seepage properties of pore structures, whereas D2 characterizes the storage capacity. D1 exhibited no correlation with D2. In comparison with D1 and D2, Dsw, which was obtained from the saturation-weighted mean of D1 and D2, exhibited better correlation with pore structure parameters than do D1 and D2. Dsw has a good correlation with rapex (pore throat radius corresponding to the apex of the plots of mercury saturation vs. mercury saturation divided by intrusion pressures), demonstrating that the segmentation feature in fractal curves is related to the intrinsic physical pore structure. The increase of the Dsw is accompanied by an increase in the displacement pressure and a decrease in the pore diameter, porosity, and permeability, thereby exhibiting poor physical properties. Dsw was found to be a more appropriate parameter than D1 or D2 for evaluating the complexity and heterogeneity of pore structures.

Modeling temperature behavior of multistage fractured horizontal well with two-phase flow in low-permeability gas reservoirs

This study aims to study the temperature behaviors and quantitatively diagnose water exits for multi-fractured horizontal wells (MFHWs) in low-permeability gas reservoirs (LPGRs) with two-phase flow. Firstly, a transient temperature prediction model is developed on the basis of mass, momentum and energy conservation with consideration of a variety of subtle heat effects (e.g. Joule–Thomson effect, thermal expansion etc.). Subsequently, synthetic cases are simulated to illustrate the temperature behaviors of a two-phase MFHW with identical/non-identical fractures. The sensitivity analysis indicates that the existing of Non-Darcy flow and gas-phase slippage effect reduces the wellbore temperature. The wellbore temperature shows significant sensitivity to reservoir permeability and fracture half-length. Then, by simulating the temperature profile of MFHWs with different water/gas ratio (WGR) of each fracture, the unique characteristics of wellbore temperature variation under each WGR situation are observed. That provides the potential to diagnose the water exits of MFHWs from temperature measurement. Finally, we apply the model to a field case and the results validated the feasibility of the developed model to simulate the temperature behavior of MFHWs in LPGRs.

Investigating the Behavior of Low Permeable Lean and Rich Gas Condensate Reservoirs

The producing behavior of low permeable gas condensate reservoirs is dramatically different from that of conventional reservoirs and requires a new paradigm to understand and interpret it. As the reservoir pressure initiates to decline and reaches to dew point pressure of the fluid then the condensate is formed and causes the restriction in the flow in the reservoir rock which results, decrease in the well productivity near the wellbore vicinity which is known as condensate blockage. Henceforward, it is better to understand the behavior of the low permeable lean and rich gas condensate reservoirs by several perspectives through the compositional simulator. Besides this study involves the following perspectives; the increase in the number of wells and by varying the flowrate of the gas in six different cases for low permeable lean and rich gas condensate reservoirs. It was concluded that low permeable lean and rich gas condensate reservoirs have similar gas recovery factors. Whereas the CRF plays inverse behavior for both reservoirs as CRF is maximum for lean gas condensate at single producing well but for rich gas condensate reservoir the CRF increases as the number of wells escalates. Additionally, in second effect the varying gas flowrates lean gas condensate reservoir has maximum CRF at lesser flowrate but it is opposite for the low permeable rich gas condensate reservoir, for single or two producing wells the flowrate effect plays but when the number of wells is increasing there is not any significant change in CRF

Guest Editorial: Growth in Unconventional Resources Adds to Technical Literature

Guest editorial Unconventional resources require technologies typically not used to produce conventional oil and gas reservoirs. In particular, starting in the late 1990s, the combination of horizontal wells and multistage hydraulic fracturing has been key to rapidly increasing success in exploiting unconventional resources. While the well completions in unconventional resources may be more expensive than those typically applied in conventional reservoirs, the advantage is access to the very large volumes of these resources, which include tight (low-permeability) gas in sandstone or carbonate rock, coalbed methane, and organic-rich source rocks classed as shale gas and tight oil. The exploitation of shale gas, along with tight gas and coalbed methane, enabled a stunning increase in US natural gas production of approximately 4 Tcf/D from 2005 to 2010. A similar story followed a few years later for US crude oil production. Tight oil production ramped up nearly 5 million B/D over approximately 5 years, starting in 2008, and continues to rise. And it is not just the US that is benefiting—unconventional resource development is now beginning to spread to other countries around the world. The technologies that accomplished these near miracles were developed, implemented, and continue to be improved largely by SPE members, and this has been reflected in an upsurge in conference presentations and journal publications related to unconventional resources over the past decade. This is illustrated by the chart (Fig. 1), which shows a dramatic increase in the percentage of peer-reviewed papers related to unconventional resources published in SPE Reservoir Evaluation & Engineering (SPEREE) beginning in 2011. Before 2011, there were only a handful of publications related to unconventional resources each year, primarily dealing with coal-bed methane and fundamentals of fluid flow in tight pore systems. Beginning in 2011, publications related to the analysis and prediction of shale gas reservoir production performance showed a sharp increase, followed a few years later by publications related to tight oil reservoir performance analysis and prediction. In addition, enhanced oil recovery from tight oil reservoirs has become an increasing area of focus. This year, over half of the publications in SPEREE will be related to unconventional resources. As further indication of the increasing importance of unconventional resources, in July SPE released Unconventional Gas and Tight Oil Exploitation, the first SPE monograph devoted exclusively to the topic.