Carbonate rock holds 60% of the global oil and gas reserves, but they are becoming more and more expensive and difficult to develop. With large reservoirs maturing, operators are forced to explore and produce from deeper resources, which are tight, highly stressed, and under high temperature. In today’s economic environment of USD 50/bbl, the cost of extracting hydrocarbon from these reservoirs needs to be scrutinized to maximize profitability. This means increasing drainage of wells using effective stimulation and optimizing production profile along the well.
Generically, carbonate matrix stimulation means pumping acids, retarded or unretarded with various functional additives, through coiled tubing or by bullheading, followed with diverting agents. Treatment volume and injection rate are based on rules of thumb formed from experience accumulated in the industry and based on laboratory studies with cores under achievable experimental conditions. Success is evaluated from the pressure and temperature response during treatments or incremental production increase after stimulation. There has been ongoing development to add science and engineering to the art of matrix stimulation so that the fluid volume can be reduced, the placement can be more controlled, and the result can be measured more directly and reliably. Adapting multiple operations in a single trip, by combining mechanical tactics such as jetting and chemical tactics such as energized or in-situ-generated acid, further allows time and cost saving while minimizing risks and enhancing well productivity. Measuring real wormhole penetration in the reservoir will help complete the loop of design, execution, and evaluation. It adds tremendous value for engineers to optimize the matrix treatment. Alternative physics and chemistry are on the horizon as well. Temperature-induced fracturing is one idea to increase well and reservoir connectivity, although the ability to control the depth of fracturing still needs to be worked out. It might help operators tailor stimulation in particular reservoir and geological properties for bypassing near-wellbore damage.
Tight formations are candidates for hydraulic fracturing as the default. However, the solubility of carbonate by various chemicals provides opportunities to extend the well drainage radius effectively without the intensive equipment, material, and infrastructure demand of hydraulic fracturing. The industry has developed significant portfolios of technologies for stimulating carbonate reservoirs, covering intervention tools, pumping processes, chemicals, and diagnostic mechanisms. Standing alone, no technology can deliver productivity optimization and maximized cost-effectiveness. Integrating technology-provider ingenuity with operator knowledge in applicability will be key to using these technologies to make matrix stimulation more effective in delivering hydrocarbons from the increasingly difficult-to-tap resources.
Recommended additional reading at OnePetro: www.onepetro.org.
SPE 181845 Far-Field Diversion in Hydraulic Fracturing and Acid Fracturing: Using Solid Particulates To Improve Stimulation Efficiency by Vanessa Williams, Baker Hughes, et al.
SPE 183465 A Novel Approach for Stimulation of a Heterogeneous Thin-Layered Reservoir in an Offshore Field, Abu Dhabi by S.F. Nofal, Abu Dhabi Marine Operating Company, et al.
SPE 181823 A New Acid-Fracturing-Fluid System for High-Temperature Deep-Well Carbonate Reservoir by Ying Gao, China National Petroleum Corporation, et al.