Production performance analysis for deviated wells in composite carbonate gas reservoirs

The strong heterogeneity of carbonate gas reservoir makes the formation exhibit composite properties. The inner region adjacent to deviated well is full of matrix, natural fractures and vugs, while the outer region contains matrix only. It is very challenging to incorporate geological features of carbonate reservoirs and evaluate the production performance of deviated wells. This paper presented a semi-analytical model to study the pressure behavior and production performance of deviated wells in composite, anisotropic and stress-sensitive carbonate gas reservoirs. In inner region, the interaction between matrix, vugs and fractures can be described by triple-porosity/single-permeability model, in which primary flow occurs only through the fractures, and the outer region can be represented by single porosity media. The stress-sensitive power exponent is proposed and estimated through laboratory experiments and curve fitting, and pseudo-pressure and pseudo-time are introduced to consider this effect. Laplace transformation, Fourier transform and inverse, Stehfest inversion algorithm and point source function are used to calculate the well bottom-hole pressure and production rate. When the inclination angle is equal to 0° approximately, the rate-transient curves of this model match very well with the conventional vertical well model. In addition, the accuracy of the model is validated by comparing the pressure response with monitoring data collected from a deviated well in Gaoshiti-Moxi carbonate gas reservoir. A synthetic case is utilized to analyze the effects of stress-sensitive power exponent, inner region radius, fracture horizontal permeability, horizontal-vertical permeability ratio and inclination angle on gas well production performance. Through the sensitivity analysis of relevant factors, we come to some conclusions that a large stress-sensitive power exponent has negative impact on well performance; a large inner radius, a high fracture horizontal permeability and a large inclination angle can heighten gas production rate in a short period, while it also leads to the drastic declination of production rate; a large horizontal-vertical permeability ratio which means the formation has strong anisotropy can significantly reduce gas production rate. With its high efficiency and simplicity, this semi-analytical approach will serve as a useful tool to evaluate the well productivity and pressure behavior for composite carbonate gas reservoirs.