Productivity of Hydraulically Fractured Calcite Rich Shale Reservoirs Enhanced by Acid

Abstract

The current low oil prices have highlighted the need for enhancement to current production from shale formations. Prolonging the production life and increasing productivity of the fracture network in the shale plays can be a way to improve the economics. Many shale wells, drilled and completed based on current completion strategies, show sharp decline after initial flash production. One of the reasons behind this can be attributed to closure of partially propped secondary fractures, proppant-less tertiary, and natural fractures. These fractures, which do not have multiple layers of proppant collapse under increasing effective closure stress as the reservoir depletes. The healing of these fractures limits the stimulated reservoir volume (SRV) to mainly the primary fractures. To keep fractures conductive under higher closure stress with reservoir pressure depletion, injection of reactive fluid after proppant fracturing is postulated. The etching pattern developed using reactive fluids can enhance fracture conductivity of secondary, tertiary and natural fracture systems. To establish the effectiveness of proposed method, experimental work was conducted using 2 and 15% HCl on calcite rich Eagle Ford shale samples. To scale up the results to reservoir level, CGM IMEX model with simulated SRV surrounding a horizontal lateral was used. The fracture conductivities obtained during the experimental work were assumed in the model to predict the improvement in ultimate recovery. 2% HCl treatment showed significant improvement in productivity when compared with non-treated samples or samples treated with 15% HCl. The fracture conductivity improved three times in propped (primary) fracture and six times in fracture without proppant (tertiary) post acid treatment with 2% HCl, however partially propped (secondary) fracture showed decrease in fracture conductivity. Proppant embedment and high stress concentration expected in case of partial proppant monolayer are probably main reasons of the drop in conductivity. In spite of loss in fracture conductivity for these partially propped fractures, simulations showed 3% improvement in recovery factor after stimulation with 2% HCl. Obviously, there will be proppant embedment in the main fracture, but it was less significant than the ones observed in the secondary fractures. The experimental and simulation work both indicate that application of low strength acid in conjunction with proppant fracturing may further increase effectiveness of slick water fracturing in tapping hydrocarbons from unconventional plays. Combining low strength acid along with slick water fracturing not only has potential to improve the initial production rate and extent of reservoir which can be drained through a single well bore; but also has capability to prolong the reservoir productivity.

Keywords: Acid, fracturing, reactive fluids, shale reservoirs, unconventional, stimulation