Efficient CO2 Multistage Acid Stimulation in Deep Hot-Gas Reservoirs

Acidizing/acid fracturing is an established method of production stimulation in carbonate reservoirs. Over time, reservoirs become depleted, gas production declines, and flowback initiation can require additional time and costs. Energizing/foaming the stimulation fluid was determined to efficiently improve stimulation results and enhance post-treatment well cleanup. The gases most commonly used to energize the treatment fluids include nitrogen (N2) and carbon dioxide (CO2). This paper presents and discusses the results of an effectiveness study of foamed acidizing treatments performed using multistage completions; successful production outcomes were achieved in depleted reservoirs. The Design-Execution-Evaluation (DEE) cycle begins with a Multistage Fracturing (MSF) well evaluation to determine whether the well is a candidate for foamed treatment. This evaluation includes studies of the reservoir data (reservoir pressure, lithology, and permeability), completion data [including fracture ports and openhole (OH) packers placement versus hole size, reservoir net pay, and lithology], and offset wells stimulation results. Foamed MSF treatments are designed to help enhance post-stimulation performance regarding cost, operational efficiency, and completion limitations. Post-job evaluation includes highlighting the treatment as well as production analysis using a numerical simulator. The post-job evaluation also serves as an input to the design of upcoming treatments. Foamed multistage acidizing using CO2 foam proved to be successful in terms of post-treatment kickoff, cleanup, and production compared to conventionally treated wells. This success can be attributed to the following effects: Faster post-fracture cleanup as a result of the decreased liquid volume pumped into the reservoirHigher productivity resulting from acid placement benefits enabled by introducing CO2 foam (diversion, retardation)Well delivery time reduction because the post-treatment N2 lift was eliminated as a result of CO2 energy stored in the wellbore and the reservoir This paper presents the study of CO2 MSF treatments compared to conventionally treated MSF wells. The study results can be used to further optimize treatment designs and improve field execution of upcoming MSF operations as well as help reduce overall well delivery time.