Determining Hydraulic Fracturing Treatment Methods for Wells in Deep and Tight Carbonate Reservoirs

ABSTRACT: The development of deep and tight carbonate reservoirs is essential due to their widespread distribution and significant hydrocarbon accumulation. Acid fracturing is commonly employed to stimulate wells in these reservoirs, offering advantages such as ease of application and avoiding screen-out issue compared to proppant fracturing. However, acid fracturing may face challenges in maintaining effective fracture conductivity, particularly under high in-situ stress conditions typical of deep and tight reservoirs. Relying solely on acid fracturing can potentially lead to reduced oil and gas production. To address this challenge, we propose an integrated workflow for optimizing stimulation in deep and tight carbonate reservoirs. This workflow assists in selecting the appropriate fracturing method by incorporating several key components: carbonate rock typing, calculation of the breakdown pressure envelope and optimal perforation direction, estimation of acid fracture conductivity, identification of ideal perforation locations, and determination of fracturing methods (acid or proppant fracturing). To evaluate acid fracture conductivity and its potential decline, we introduce two new parameters: the acid fracture conductivity threshold (AFCT) and the fracture conductivity declining index (FCDI). These parameters, which can be derived from log data, 1D Mechanical Earth Model (MEM), and laboratory tests, help assess the performance of acid fracturing. The AFCT measures the effectiveness of acid fracturing, while the FCDI accounts for the impact of in-situ stress and rock compressive strength on fracture conductivity. The workflow recommends using acid fracturing when the AFCT is high and the FCDI is low, indicating favorable conditions for acid treatment. Conversely, it suggests proppant fracturing when the AFCT is low or the FCDI is high. An example study demonstrates the application of this workflow to select the appropriate fracturing method for wells in deep and tight carbonate reservoirs. By analyzing log data to identify optimal rock types and perforation locations, the workflow provides guidance on whether acid or proppant fracturing should be employed. Overall, proppant fracturing seems to be a better choice for deep and tight carbonate reservoirs compared to acid fracturing.