Stress-sensitive Gas Reservoir Research Articles

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.

Hydromechanics Model of Natural Gas Transport in Stress-sensitive Dual-porosity Medium

The assumption of constant permeability is not strictly applicable to porous media where rock properties undergo changes, such as stress-sensitive gas reservoirs. This study developed a hydromechanics model of a horizontal well in dual-porosity media considering the stress-sensitivity effect. The solution methods include variable substitution, perturbation technique, Laplace transformation, Sturm-Liouville eigenvalue theory, orthogonal transformation and numerical inversion. The results showed that the production rate decreases with the increasing of permeability modulus and the derivative curve will be warped up during the whole system late radial flow stage. The research has both theoretical and practical significance in predicting production performance and evaluating underground fluid transport in such formations.

Productivity Analysis Method of Abnormal High-Pressure Gas Reservoir in Ying-Qiong Basin

Ying-Qiong Basin in the west of South China Sea contains plenty of abnormal high-pressure gas reservoirs, whose stress sensitivity is crucial for well productivity. To explore the influence of stress sensitivity on production, the variable outlet back pressure stress sensitivity experiments were applied to test core sample permeability under different burden pressure and obtain the relational expression of power function of core stress sensitivity. Afterwards, new productivity equation is deduced in consideration of reservoir stress sensitivity, and the affection of stress sensitivity on production is analyzed. The result demonstrates close link between stress sensitivity and productivity, since single well production decreases dramatically when reservoir stress sensitivity has been taken into account. This research is constructive for well-testing data interpretation in stress sensitive gas reservoirs.

Transient analysis for fractured gas wells by modified pseudo-functions in stress-sensitive reservoirs

In order to study the effect of stress sensitivity on transient flow behavior for fractured wells, this article demonstrates how to account for stress sensitivity effect in gas reservoirs using modified pseudo-functions approach. By making this modification, the governing equation can be linearized. This study presents production performance of gas well for the most widely used inner conditions: constant flow rate and constant bottom-hole pressure. Furthermore, type curves are generated to investigate the effects of stress sensitivity, reservoir size and fracture properties (i.e., conductivity and half-length). Calculative results show that a bigger stress-sensitivity coefficient will lead to higher degree of bend upward on the log-log pressure curves, indicating bigger pressure depletion and rate decline in the late-time period; reservoir size mainly affects the duration of pressure depletion and rate decline; fracture conductivity and fracture half-length affect pressure and production in the early-time period. This study provides comprehensive analysis of stress sensitivity for fractured gas wells and new insight into investigating production performances in stress-sensitive gas reservoirs.

Well Test Interpretation Method of Horizontal Well in Stress-Sensitive Gas Reservoir

Stress-sensitive widely exists in fractured reservoir. In this paper, a mathematical model of flow in stress-sensitive reservoir with horizontal well is established based on experimental data and with process of linearization. By using of Lord Kelvin point-source solution, Bessel function integration and Poisson superimpose formula, the dimensionless pressure response function of horizontal well in infinite stress-sensitive reservoir is obtained. And then the derivative type curve is calculated. Based on the type curve, the characteristics and influencing factors of the fluid flow through porous medium of horizontal well in stress-sensitive gas reservoir are analyzed.

Transient Tests and Long-Range Performance Predictions in Stress-Sensitive Gas Reservoirs

This paper investigates the use of a simplified stress characterization in the real-gas-potential expression and the application of "succession of steady states" techniques for test analysis and performance prediction in stress-sensitive reservoirs. The technique is shown to give good results with numerical data and with data from a laboratory model. Introduction Rising prices and declining reserves have stimulated interest in deep, low-permeability gas reserves and have added a new dimension to the interpretation of transient pressure tests. How does the well-test engineer or pressure tests. How does the well-test engineer or reservoir engineer recognize and account for stress-sensitive permeability? Fatt et al. measured this stress sensitivity permeability? Fatt et al. measured this stress sensitivity about 25 years ago and several others have presented data during the intervening years; fortunately, however, the increase in effective overburden pressure with declining reservoir pressure is not an important flow-reducing mechanism for most wells. If we consider the net confining pressure or overburden pressure to be the total overburden pressure minus the pore pressure (an assumption we will justify later), Fig. 1 will show why this is true. A normally pressured reservoir at 6,000 ft would have a net confining pressure of about 3,000 psi at discovery and 5,500 psi at abandonment. The top curve in Fig. 1 shows a permeability change of 4 percent (from 0.81 to 0.78) and the lower curve shows a change of 14 percent (from 0.58 to 0.50). The latter number is becoming percent (from 0.58 to 0.50). The latter number is becoming significant and permeability changes near the wellbore would be even greater; however, when this effect is compared with other uncertainties over the 15- to 20-year life of a gas reservoir, it probably can be ignored. However, recent data on low-permeability gas sands show much greater permeability reductions (see Fig. 2). This, along with the possibility of abnormal pressures shifting the initial net confining pressure toward the left, creates a situation where permeability changes cannot be neglected. For example, if a reservoir represented by Fig. 2 was discovered with poB= 10,000 psia, pi= 8,000 psia, and pub= 1,000 psia, the change in confining pressure would be from 2,000 to 9,000 psia over the life of the field. The permeability change would be several-fold (0.6 to 0.2 for Core I and 0.3 to 0.05 for Core H). Vairogs and Rhoades have used a numerical model to demonstrate the errors introduced into transient test analysis and long-term performance predictions. This paper attempts to simplify their theory and to eliminate paper attempts to simplify their theory and to eliminate the need for core data and a numerical model for making performance predictions. Data are presented from a performance predictions. Data are presented from a numerical model and a laboratory model to support the suggested techniques. Theory Three factors must be considered:a mathematical characterization of stress sensitivity,a nondimensional means for representing what is basically a threedimensional stress problem, andmodification of the real gas potential to include the stress-sensitive permeablility. permeablility. Permeability Characterization Permeability Characterization If we are to avoid the expensive process of obtaining laboratory, core data for every well, we must look for a means for characterizing a typical permeability reduction curve. Wyble suggested that the curves represent an exponential decline approaching a limiting value. JPT P. 1025