Transient Pressure Behavior of Acid Fracturing Well in Tidal Flat Carbonate Reservoirs

Abstract

Abstract Different from the marine carbonate reservoir, Tidal Flat Carbonate Reservoir (TFCR) rock contains Natural Fracture (NF) and Natural Dissolved Vugs (NDV). The NDV are usually distributed alone or along the NF in the rock core, which can be observe by the eyes. In the process of acid fracturing, Acid Fracturing Fracture (AFF) will activate NF and crack Carbonate Rock Matrix (CRM), acid fluid will etch NF and NDV. Finally, a complex Acid Fracturing Fracture Network (AFFN) with high conductivity is formed in Acid Fracturing Region (AFR). However, the range of the AFR is limited, there are still a lot of NF and NDV in Carbonate Rock Region (CRR). The porosity and permeability of CRM are very low, the distribution pattern of NF and NDV are very complex. When pressure through the AFR and spread to the farther reservoir rock, the pressure behavior begins to show the multiscale flow characteristics. Therefore, the Non-Darcy Flow (NDF) of CRM in CRR and the Stress Sensitivity (SS) of AFFN in AFR need to be considered. Unfortunately, low porosity and low permeability in TFCR are seldom considered in typical transient pressure behavior model, such as NDF caused by Threshold Pressure Gradient (TPG) and SS caused by reservoir pressure decrease. This paper presents a new transient pressure analytical model in TFCR, where the reservoir flow is subdivided into two flow regions: AFR and CRR. The effect of NDF in CRR and SS in AFR are both considered. Laplace transformation, perturbation method, Green function and Stehfest numerical inversion are employed to solve the model comprehensively. The model solution is verified with real transient pressure test data in TFCR. The effects of related influential parameters on transient pressure behavior, such as NDF, SS, acid fracturing area, rock property improvement degree and flow channel improvement degree. Field example is conducted to shown the applicability of the new model, applied to the actual TFCR, the actual Bottom Hole Pressure (BHP) data of several acid fracturing wells pressure buildup test is well fitted. The results of this study can be helpful to better understand the flow mechanisms and transient pressure behavior of acid fracturing well in TFCR, which can be also used to evaluate acid fracturing measure and transient production performance. The judgement and recognition of the acid fracturing area and improvement degree have guiding significance for the repeated acidizing or hydraulic fracturing in the later stage of increasing production.