Hydraulic Fracturing Tight Reservoirs: Rock Mechanics and Transport

Abstract

Conventional reservoirs have been fracture stimulated using acid fracturing and proppant fracturing. Acid fracturing is performed to improve well productivity in acid-soluble formations such as limestone, dolomite, and chalk. Hydrochloric acid is generally used to create an etched fracture, which is the main mechanism for maintaining the fracture open during the life of a well. Proppant fracturing is an alternative option that has been applied in carbonaceous and siliceous formations. There is no quantitative method to provide an answer of whether acid fracturing or proppant fracturing is an appropriate stimulation method for a given carbonate formation. How rock mechanics can be applied to decide on what method is more effective? Laboratory experiments have been performed to simulate acid etched to study the effect of elastic, plastic and viscoelastic rock behavior and their effects on fracture conductivity. Comparison of acid vs. proppant fracturing conductivity in carbonate formation is presented.Fracturing low permeability reservoirs is totally different than fracturing tight formations. The fracture geometry required in low permeability reservoirs need to be planar, conductive and penetrating deep in the reservoir. Fracture complexity in these reservoirs is to be avoided for optimum stimulation treatment. However, in fracturing tight formation, a complex fracture network is desirable for better recovery. Creating multiple fractures in horizontal wells without the use of mechanical intervention, is becoming essential especially in tight gas reservoirs. We have learned how to initiate hydraulic fractures into a specific direction and place as many fractures as desired in horizontal wells but with casing and perforation. The challenge now is to initiate weak point across the horizontal well such that fracturing fluid will initiate a fracture there. How rock mechanics has been applied to achieve this objective? We are fracturing tight gas sand in harsh environment, at depth more than 18000 ft, of temperature close to 400 °F, and one can figure out the extreme in-situ stresses relevant to this depth. When the reservoir pressure decreases, the elastic displacement in response to the increase in effective stress will cause natural fractures to close leading to a decline in reservoir productivity. The matrix medium feeds the natural tensile fractures which carry the fluids to the wellbore. The decline in conductivity with increasing effective stress should follow a logical declining rate to support a given production rate. How the concept of effective stress has been applied to understand the stress-dependent conductivity of various conductive components of a given reservoir? Rock mechanics testing of these stress sensitive reservoirs becomes vital to optimize fracturing tight formations. Economical production from tight reservoirs, including shale gas and shale oil formations, requires horizontal well drilling and massive proppant hydraulic fracturing stimulation. The stimulation involves generating sufficient fractures network or stimulated reservoir volume (SRV), which is achieved by placing optimized stimulation treatments along the horizontal section of wellbores ideally drilled from multi-well pads to increase the production rate and ultimate recovery. Hydraulic fracturing in naturally fractured formations is characterized by generating a fractures’ network that should be designed for in extremely low permeability of unconventional reservoirs. Fractures should extensively reach shale matrix to achieve commercial gas production. Therefore, production rate and ultimate recovery depend on the size of the created SRV. The transport phenomena controlling fluid flow through tight formation is no longer sufficient to be modeled by Darcy’s flow. Diffusion and imbibition are important transport mechanisms. The concept of osmosis and flow through a semi-permeable membrane component are critical. Additionally, diffusion and a special case of molecular flow due to Knudson effect will be discussed. Conventional reservoir simulation collapses when trying to simulate fluid flow through tight reservoirs. Numerical studies on a hydraulically fractured well to simulate the dynamic processes during fracturing injection, following well shut-in (soaking), and production are discussed.