Low-permeability Gas Reservoirs Research Articles

Characterization of refracture orientation in poorly propped fractured wells by pressure transient analysis: Model, pitfall, and application

Wells with poorly propped fracture are often considered to be refracture candidates. However, neglecting the influence of unpropped segments in analyzing transient pressure data from a well can produce large errors in characterization of the orientation of the refracture. To better understand the refracture orientation, we derived a semi-analytical model for pressure-transient analysis of poorly propped fractured wells intercepting reoriented refracture in low-permeability gas reservoirs. Our initial fracture-flow model considered unpropped segments with lower conductivity than propped segments. The resulting solutions display underlying characteristics (notably on log-log plots); these characteristics will be especially useful to determine the refracture orientation. We observed that the transient pressure behavior during specific flow regimes will be distorted by poorly propped fracture. The paper discusses the effect of refracture orientation on transient pressure behavior, and further examines the pitfalls in the characterization of the orientation of poorly propped fracture wells. The results can provide a benchmark to estimate fracture parameters in field applications. In addition, we simulated a synthetic case to develop the relationship between refracture orientation and pressure responses during pseudo-boundary-dominated flow. This relationship provides a quantitative basis to directly estimate the refracture orientation by analyzing transient pressure data.

New rate-decline forecast approach for low-permeability gas reservoirs with hydraulic fracturing treatments

Estimating production in low-permeability gas reservoirs accurately has been of more concern to oil and gas industries in recent years. Until now, various techniques to estimate well production have been developed. Among them, decline curve analysis models have been recognized as the most efficient and easiest approach to be applied. Unfortunately, each decline curve model has its own limitation and will not allow us to forecast production in low-permeability gas reservoirs with confidence. In this paper, firstly, the review of Arps, stretched exponential production decline (SEPD), and Duong methods is conducted explicitly to illustrate the reasons resulted in their limitations. Subsequently, a new empirical decline model with two fitting parameters is developed based on the proposed linear relation of logarithmic production versus the product of square root of time and logarithmic time, which takes the change of decline exponent with production time into account. In addition, a detailed procedure applying this novel decline model for low-permeability gas wells is presented, which is reliable and easy to be utilized. Furthermore, this new model is validated by numerically simulated cases and field observations, which appears good match between forecast rates calculated by this proposed method and numerically simulated rates/field rates. The comparison between this novel method and several traditional methods are further conducted, which indicates that this new approach leads to more reliable forecast than commonly utilized approaches. This work should provide a theoretical guidance to assist analysts in estimating hydrocarbon production rapidly and efficiently in low-permeability reservoirs.

Old well sidetracking selection standards: Sulige low-permeability gas field, China

The old well sidetracking technology is an effective means to stabilize production and increase recovery in later stage of gas field development. Based on economical and reservoir geologic parameters of Sulige low-permeability gas field in China, method and procedure of old well sidetracking selection standards are established, which can effectively improve the sidetracking effect and adequately develop remaining gas. Comprehensively considering corresponding economic parameters including sidetracking drilling cost, operating cost, management cost, gas production rate and gas price, recoverable reserve standard of sidetracking well is determined first. According to gas recovery, gas-bearing area, porosity and gas saturation, standards of remaining geological reserve, reserve abundance and gas pay thickness of sidetracking well are obtained. According to extension direction and distribution characteristics of the sand body in research area, numerical simulation conceptual model of typical sidetracking horizontal well is established, and then standards of old gas well production rate and gas cumulative production rate for sidetracking are obtained. According to the standards, well Su 36-6-9CH is selected to be sidetracked as a horizontal well and its development effect is good. The method has important value of popularization and application in similar low-permeability gas reservoir.

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.

A Complex Network Model for Analysis of Fractured Rock Permeability

Fractured rocks exist widely in nature. The fracture network is an effective storage space and main seepage channel of low‐permeability oil and gas reservoirs, which controls the seepage system of low‐permeability oil and gas reservoirs. The connection characteristics of fracture networks are complex and evolve dynamically with time. The rise of complex network research can provide reliable analysis for the relationship between network structures and network behaviors. In this work, the fracture network is considered as a hierarchical network with self‐similarity, and complex network theory is applied to analyze the permeability of fractured rocks. According to the power‐law relationship of degree distribution of network nodes, the number of nodes is corresponding to the number of network edges and a new power‐law distribution relationship of edges with degree of nodes is proposed. Eventually, the permeability model of fractured rocks is derived and it is found that permeability of fractured rocks is a function of degree of maximum node kmax, self‐similarity index γ, power index dk, and other structural parameters. Compared with the existing numerical simulations, the validity of the model is verified. By calculating the influence of model parameters on the permeability, the following results are obtained: (1) fracture porosity is directly proportional to permeability; (2) fracture surface density is linearly increasing with permeability; (3) power index is inversely proportional to permeability; and (4) permeability is exponentially increasing with the maximum degree of a node.

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.

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.

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.

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.

Investigation of temperature behavior for multi-fractured horizontal well in low-permeability gas reservoir

This study aims to interpret the temperature behavior of a cemented multi-fractured horizontal well (MFHW) in a low-permeability gas reservoir (LPGR) during production. First, considering heat conduction, heat convection, thermal expansion, viscous dissipation, and the Joule–Thomson effect, a comprehensive numerical temperature prediction model is developed under a single-phase condition. The developed models are formulated for the reservoir and wellbore domains based on mass, momentum, and energy conservation. The non-Darcy law is applied to the numerical models, and radial flow in the hydraulic fractures is accounted for when the reservoir and wellbore models are coupled. These developed models are solved numerically by the finite difference method. Then, synthetic cases demonstrate the models’ ability to predict the temperature behavior and clarify the change regularity of the wellbore temperature profile for an MFHW in an LPGR. The effects of pressure interference among hydraulic fractures on the inflow rate are analyzed. Based on the sensitivity of arriving temperature to the fracture parameters, an approach to plotting fracture parameter diagnosis charts are introduced. In addition, a field case is provided to illustrate the application and feasibility of the new models on the basis of the accurate simulated results of wellbore temperature profiles.

Novel Method of Production Decline Analysis

ARPS decline curves is the most commonly used in oil and gas field due to its minimal data requirements and ease application. And prediction of production decline which is based on ARPS analysis rely on known decline type. However, when coefficient index are very approximate under different decline type, it is difficult to directly recognize decline trend of matched curves. Due to difficulties above, based on simulation results of multi-factor response experiments, a new dynamic decline prediction model is introduced with using multiple linear regression of influence factors. First of all, according to study of effect factors of production decline, interaction experimental schemes are designed. Based on simulated results, annual decline rate is predicted by decline model. Moreover, the new method is applied in A gas filed of Ordos Basin as example to illustrate reliability. The result commit that the new model can directly predict decline tendency without needing recognize decline style. From arithmetic aspect, it also take advantage of high veracity. Finally, the new method improves the evaluation method of gas well production decline in low permeability gas reservoir, which also provides technical support for further understanding of tight gas field development laws.