Recognizing the Impact of Hydraulic Expulsion Fracturing on Reservoir Pressure in Mud-Rock Reservoir Systems

Sturm, Stephen ¹
(1)Schlumberger DCS, Denver, CO.

The development and preservation of overpressured compartments in unconventional shale reservoirs requires low-permeability seals that inhibit fluid movement of generated hydrocarbons. As hydrocarbons are generated from kerogen, fluid pressure exceeds the tensile strength of the seal and subsequent hydraulic expulsion fracturing will occur. Overpressured systems are maintained by thick competent seal rocks that are inhibit fluid migration from the pressure cell. In mud-rock reservoirs dominated by vertical lithologic heterogeneity, expulsion fracturing and the associated release of pressure may be episodic (burping) or occur as a single event. Resulting fracture density and fracture heights are an imprint on the efficiency of either equilibration or maintenance of pressure compartmentalization within the reservoir.

Hydraulic expulsion fractures are characterized as near-vertical planes. They are bed or laminae-bound and calcite-filled. They are distinguished by occurring in relatively structurally-flat (<2°) undeformed strata with varying fracture density. They are most common in the organic source rock and the sealing, conventional siltstones superjacent to the source rocks. A higher density of expulsion fractures usually occur in kerogen-rich heterogenic rocks with thin incompetent internal sealing rocks. As the column of rock is buried, the kerogen-saturated column matures and generates hydrocarbons. These hydrocarbons fill the pores within the kerogen, and the pore pressure increases due to the very low permeability of these nano-scale pores. Ultimately, the in-situ pore pressure exceeds the tensile strength of the sealing laminae, and gas is burped into contiguous beds. As net pressure in the reservoir is reduced, CO2 in solution decreases and CaCO3 precipitates in the fracture voids. The orientation of these fractures is therefore aligned with the principal horizontal stress field (S_Hmax) at the time of expulsion and may, or may not be the current principal stress orientation.

These intense expulsion-fractured, normally pressured mud-rock reservoirs are generally found to be sub-economic targets. The majority of commercial mud-rock reservoirs are considered transitional between over-pressured and normal pressure. While hydraulic expulsion fractures are present in these economic mud-rock reservoirs, they occur in lower densities. Because of this, significant pore-pressure has not been released from the system and economic viability is maintained.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.