Insights from an Extensive Laboratory Geomechanics Characterization Study
Long-term compaction and failure behavior of a large field, offshore Abu Dhabi
Following decades of production from multiple separated stacked reservoirs, a maturing field has undergone many subsurface activities, such as drilling, oil and gas production, and injection of water and gas for reservoir stimulation. Considering the long-term field development plan, one reservoir will be depleted by 5,000 psi after 20 years. Such high levels of depletion can produce severe reservoir compaction and pore collapse, leading to a rapid loss in permeability, generation of fines (byproducts of pore collapse and/or grain crushing), subsidence, wellbore instability, damage to well completion integrity, and loss of caprock containment. An extensive rock mechanics laboratory study was conducted to assess the possibility of pore collapse and prevent and mitigate risks proactively from adverse reservoir compaction.
During depletion, the reduction in reservoir pressure results in unequal increases in vertical and horizontal effective stresses and thus an overall increase in the effective mean and shear stresses on the reservoir. At reservoir pressures below a critical value (obtained via laboratory testing or post-failure field analysis), the reservoir may compact at accelerated rates. To fulfill the objective of this study, a series of tests were designed to probe all possible depletion scenario.
Rock failure parameters were evaluated through a sequence of tests of carefully selected, representative samples. Failure envelopes defining shear (dilatant) and compaction ("cap") for compactable sediments are often strongly nonlinear. For field applications, it is useful to provide a visualization of the preproduction-state in-situ stress conditions and the possible stress path trajectories of the reservoir (from triaxial Ko=0 to hydrostatic Ko=1) as a function of reservoir depletion. Using this display, the level of depletion resulting from accelerated compaction was identified through laboratory testing. Tests conducted for assessment of reservoir compaction are: uniaxial- strain compression (far-field compaction), triaxial compression (near-wellbore compaction), hydrostatic (define the compactant cap), and constant stress-path (fixed Ko, far-field compaction).
The rock units evaluated were exceptionally heterogeneous, with tensile strength and unconfined compressive strength ranging from 323 to 2,987 psi and 2,944 to 34,481 psi, respectively. Testing conducted on the reservoir intervals were designed to capture all possible depletion scenarios during the potential life of the reservoir. Results have shown that rock with porosity >26% have a propensity for accelerated compaction prior to plan abandonment pressures. Further, accelerated compaction does not occur for rock with porosities below 25%, even following extreme reservoir depletion of 5,000 psi. This paper outlines core analysis workflows that can adequately assess potential changes to reservoirs during depletion—from preproduction conditions to abandonment. Further, the paper highlights the importance of understanding rock heterogeneity prior to initiating any core analysis program.