DETERMINING IN SITU PROPERTIES OF CAPROCK FORMATIONS USING PORE PRESSURE RESPONSES FROM GROUTED-IN PRESSURE TRANSDUCERS

Caprock is an inclusive name for any overburden rock formation that may trap or concentrate oil, gas or water, and prevent it from migrating into overlying shallow aquifers or to ground surface. Typically, caprock formations consist of low permeability shales with a high clay content (also known as claystone aquitards). The ability to characterize the caprock properties and subsequently determine caprock integrity is critical to successful thermal recovery of petroleum products using methods such as Steam-Assisted Gravity Drainage and Cyclic Steam Stimulation. The formation properties (compressibility, α; specific storage, S_S; and hydraulic conductivity, K) of claystone aquitards are traditionally determined using laboratory tests; however, core samples removed from the subsurface may yield results not representative of in situ conditions. An alternative method is presented here using grouted-in pressure transducers to estimate in situmatrix properties by monitoring pore pressure responses.

Loading efficiency, S_S, and α were estimated by analyzing pore pressure responses to known barometric responses, and K was estimated from the recovery of the pore pressure after installation to stabilization (0.5-5 months). This method was successfully applied at multiple sites (predominantly in the Williston Basin) for over 50 transducers in 11 boreholes completed to various depths (100-325m BG). 1D consolidation (n= 8) and triaxial (n = 14) laboratory tests were conducted to compare with the in situestimates. Results show that the laboratory tests overestimate α of the formation and underestimate K of the formation by up to one order of magnitude in both cases.

The pore pressure records provide evidence that barometric pressure, earth tides, and moisture loading can be observed from surface to 325 m BG (theoretically deeper). The findings suggest that the fully grouted method can provide an accurate and reliable means to monitor pore pressure changes in shallow and deep aquitard systems, better define in situ parameters for overconsolidated argillaceous deposits, and help characterize caprock integrity. Furthermore, multiple pressure transducers in a single borehole allow for the estimation of in situ parameters at various depths without having to drill multiple boreholes to achieve the same depth profile.