Study Quantifies Stress Sensitivity of Fractured Tight Reservoirs
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Wells in unconventional reservoirs can experience sharp rate declines in the early stage of production, especially when experiencing aggressive drawdown. One key factor affecting rate decline is rock sensitivity to increasing compressive stress. The complete paper describes and quantifies the stress-dependence of compaction and permeability for anisotropic rock matrices, natural fractures, and hydraulic fractures, based on comprehensive rock tests of a fractured tight reservoir.
Stress Sensitivity and Drawdown
Laboratory data show that rock permeability can be reduced by 10 to 99% with increasing confining stress. Controlling factors include rock characteristics such as authigenic cementation, pore structure, clay content, natural fractures, and pore volume compressibility. Additional factors include pore throat size and shape. Low-permeability rocks are more sensitive to stress changes than are high-permeability rocks.
A direct link exists between stress-dependent permeability reduction and production decline, especially in unconventional reservoirs in which production declines rapidly during the first year. As production starts and bottomhole pressure is lowered, the effective stress on the rock near the wellbore, defined as the difference between total stresses and pore pressure, increases. Higher loading stress reduces the permeabilities (or conductivities) of induced fractures, natural fractures (or laminations), and the matrix. The increase in effective stress can lead to proppant embedment and crushing, formation spalling, fines migration, rock compaction, and closing of natural fractures. Many of these processes are irreversible and lead not only to rapid production decline but also to reduction of estimated ultimate recovery (EUR).
In 2011, analysis of a shale consortia database found that wells with restricted rate (limited drawdown) have 2 to 3 bcf higher EUR than wells with open rate (unlimited drawdown). The analysis also observed that the decline of stress-dependent permeability in propped fractures (millidarcy) is not as severe as the reduction in unpropped fractures (microdarcy) and matrix (nanodarcy).
Many unconventional fields are negatively affected by rock stress sensitivity in terms of production and EUR. Fit-for-purpose strategies have been developed to manage drawdown and minimize rock and fracture damage. The complete paper provides several examples, including the following:
- Midland Basin: A 1- to 5-psi/hour drawdown rate reduced the loading stress on the proppant pack significantly. Managed pressure drawdown also reduced water production, maintained the reservoir pressure above saturation pressure for a longer period, and mitigated sand production.
- Haynesville: Restricted drawdown was found to improve projected EUR as much as 38% over a period of 30 years of production.
- Montney formation: The operator applied downhole chokes vs. surface chokes to manage drawdown, reduce liquid loading, mitigate surface hydrate formation, eliminate sand flowback, and reduce costs. The restricted drawdown led to higher production performance and potentially increased EUR by 36% after 1 year of production.
- Utica/Point Pleasant: A managed drawdown strategy was applied with a daily pressure drop of 15 to 25 psi to sustain the production at a flat rate for approximately 1 year with an increase of 30% in EUR.
- Eagle Ford: Rate transient analysis with high-frequency data was used to develop optimal choke management and maximize well deliverability. The average 30- and 90-day cumulative productions of more than 450 wells increased 100 and 87%, respectively.
- Vaca Muerta: Operators found that an aggressive drawdown could lead to a 20% reduction in EUR. A pressure drop of 0.25 to 2 psi/hr for the initial 8 months sustained high production without fracture degradation. The timing of choke change was critical, with no substantial reduction in production after reaching pseudosteady pressure decline.
The complete paper describes a series of stress-sensitivity laboratory tests on low-porosity, low-permeability rocks with abundant natural laminations. The effect of drawdown was simulated with increases of effective confining and loading stresses. The tests include rock compaction, anisotropic permeabilities of laminated rock, permeabilities of both tensile fractures and shear and acoustic anisotropy. All fractures and laminations studied were not propped, because abundant literature exists that quantifies the conductivity of propped fractures. In addition, recent core and field experiments indicate that proppants can only reach tens of feet.
Compaction in Unconventional Rocks
Pore volume compressibility is a key property in history matching unconventional reservoir performance. Because of the low modulus of kerogen, clay, and carbonate minerals, high total-organic-content shales or tight carbonate reservoirs are more susceptible to compaction with drawdown and depletion. Furthermore, low porosity and small pores in unconventional rocks exacerbate permeability degradation. In overpressured reservoirs, grain/grain contacts are not well developed. When pore pressure is reduced, the rock matrix needs to carry higher overburden stresses. Lack of strong grains and interlocking grain contacts leads to low pore collapse pressure in many unconventional rocks.
The complete paper discusses several compaction tests in detail. To identify rocks with higher potential to fail by compaction, various mechanical properties were investigated, and the friction angle was identified as a key indicator. In Fig. 1, rock shear strength and compaction strength are defined by Mohr-Coulomb criteria and compaction caps, respectively. A typical high porosity rock exhibits a low friction angle φ1, low uniaxial compressive strength (UCS1), and high cap strength. In comparison, a low-porosity unconventional rock has a higher friction angle (φ2), and therefore higher UCS2, but low compaction tolerance (cap strength2). Combining factors such as low compaction strength; low modulus of kerogen, clay, or carbonate; weak grains; and weak grain/grain contacts in overpressured formations result in unconventional rocks possessing a high risk of compaction failure.
Stress Sensitivity of Laminated Tight Rock
Tight rocks are very sensitive to stress changes for two reasons. First, abundant laminations not only dominate rock mechanical behaviors and permeabilities, but also affect permeability anisotropy. Space between laminations tends to close with increasing confining stress. Second, because of low porosity, any small volumetric change in compaction has a significant effect on pore throats and, therefore, permeabilities.
Stress Sensitivity of Tensile (Hydraulic) and Shear (Natural) Fractures
For unconventional rock with nanodarcy matrix permeability, induced and natural fractures are the key to production and EUR. These fractures can be divided into two groups based on failure mechanisms, tensile and shear. Hydraulic fractures are tensile, and microseismic monitoring can only record shear events because of the low energy released by tensile fractures. As a result, significant discrepancies often exist between simulated reservoir volume (SRV) estimated from microseismic events and SRV estimated from hydraulic fracture geometries.
Tensile and shear fractures share two distinctive decline stages: a fast decline with closing fractures, and a steady decline with collapsing pores and a compacting matrix. Both also share a critical confining stress of approximately 2,300 psi that separates the two stages. Rapid rate decline in unconventional wells can result from the closure of unpropped natural and hydraulic fractures. The goal of drawdown management is to delay the bottomhole flowing pressure from reaching the critical pore collapse pressure.
The complete paper presents a series of stress-sensitivity tests on a low-porosity, low-permeability unconventional rock with abundant laminations. The tests have quantified the stress dependence of compaction and permeability for anisotropic rock matrices, natural fractures, and hydraulic fractures.
The study found that unconventional rocks, especially those in overpressured formations, have a high risk of compaction failure and pore collapse, regardless of their high UCS values. Contributing factors include high friction angle; low compaction strength; low modulus of kerogen, clay or carbonate minerals; and weak grains and unstable grain contacts. Three sets of test data have been used to identify rock compaction failures. Of these data sets, stress-dependent permeability is the most sensitive to pore space changes.
- For rock matrices, abundant laminations correlate with severe permeability anisotropy. Increasing confining stress results in reducing anisotropy to negligible values. Horizontal permeability declines rapidly with small increases in confining stress.
- Tensile fractures have higher initial permeability, but their permeability reduction is much faster than that of shear fractures.
- Both tensile and shear fractures decline similarly with increasing confining stress. The initial stage involves fast decline as fractures close.
Study Quantifies Stress Sensitivity of Fractured Tight Reservoirs
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