Abstract
In shale reservoirs, the stimulated reservoir volume (SRV) is comprised of multiple propped hydraulic fractures in the presence of a matrix and/or natural fracture system. Each SRV component is well known to exhibit stress dependent permeability such that there may be significant loss in permeability with increased drawdown. The concern about SRV damage due to aggressive flowback strategies has led to a variety of heuristic flowback guidelines of "x psi/day" across various operators and shale plays. However, conservative flowback strategies may also result in productivity loss due to water block and capillary trapping resulting from ineffective unloading of treatment water. Real time flowback management based on reservoir physics is therefore necessary for a balanced flowback strategy aimed at optimal SRV integrity, EUR, and well economics. Additionally, since maintaining SRV integrity is important from early transient flow to late boundary dominated flow, a flow regime agnostic approach is especially beneficial. Based on the dynamic flowing material balance, this paper presents a real time SRV integrity monitoring approach using a flow regime agnostic dynamic productivity index. Field applications for flow back diagnosis and rate acceleration are presented using examples from the Woodford, Eagle Ford, Permian, and Haynesville shale plays.
Shale reservoirs are well known to exhibit stress dependent permeability. The effective stress change results from a decrease in pore pressure and is related to the absolute stress on the rock as in equation 1 below. Stress dependent permeability is therefore synonymous with pressure dependent permeability in this paper.
(equation)
Jones (1988) presented a two-point experimental determination of the inverse relationship between matrix permeability and net confining stress for pressures up to 10,000 psi. Kwon et al. (2004) experimentally developed a cubic law correlation for matrix permeability reduction with confining pressure. Crawford et al. (2019) demonstrated via uniaxial experimental setup that that classic net confining stress approach for rock stress measurements may overestimate the permeability modulus by up to 100%. They also demonstrate permeability hysteresis typical in stress dependent permeability systems. Yilmaz and Nur (1985) had previously introduced the permeability modulus as a measure of the sensitivity of permeability to pressure change and is defined as:
