The in-situ stress, and in particular the minimum principal stress, is a major controlling parameter for many subsurface engineering issues, such as safe injection and injection pressure limitation, wellbore stability, fractured injection and stimulation, and completions optimization. In addition to these more ‘traditional’ field development decisions, in-situ stress has direct influence on the rapidly growing CCS industry, where storage volumes of CO2 are highly dependent on the initial minimum effective stress margins available in the sealing caprock(among other factors). In this work we investigate a unique in-house stress database, obtained through decades of dedicated stress testing, to better understand and quantify the relationship of in-situ stress versus depth and its relation to pore pressure. Focus is primarily on the Norwegian Continental Shelf but global results from additional passive continental margin areas are also incorporated and compared. We find that, almost regardless of the geographic area, when hydrostatic pore pressure conditionsapply, relatively simple linear relationships exist of stress versus depth and that the assumption of normally-stressed/relaxed stress regimes can be applied with a good degree of certainty. Further, where overpressure conditions are present, relationships dependent on the degree of overpressure are defined, both regionally and globally. The resulting overpressure relationship is found todiffer fromthose commonlyaccepted andused throughout industry, e.g. Breckels and van Eekelen 1982. Finally, the resulting stress trends versus depth are investigated to better identify the potential presence of high stress environments such as deeper strike-slip to reverse faulting regimes that can complicate field development decisions. While of interest to the hydrocarbon industry in general, the results of this work are highly valuable to under-explored areas where in-situ stress data is not yet available, e.g. saline aquifer prospects targeted for eventual CCS development.
Aging assets and higher depletion levels means increased challenges in terms of operations, well integrity and field development – completion and/or casing damage/collapse, restricted well access, low well pressure and restrictions in (or complete elimination of) conventional and/or safe drilling and completions windows. Nonetheless, the need to increase recovery factor and produce remaining reserves remains; thus, an ever-increasing number of fields are considering late-life development strategies where understanding the feasibility of future operational margins is critical. Through numerous related studies on this topic in recent years, we have developed an approach which focuses on the key elements affecting reservoir stress path, and subsequently, how our operational windows change with depletion/time. Focus is first given to adequate laboratory testing, specifically on uniaxial strain testing and identification of potential non-linear effects over the depletion range in question. A field-scale geomechanical modeling approach accounting for both stress dependency of elastic parameters and poroelastic effects is then implemented to consider also the potential effects of geometry and material heterogeneity. Last, supported by recent operational experiences, updated best practice suggestions with regards to fracture gradient considerations in depleted sections are implemented as direct model output, specifically for the future wellpaths and/or areas to be developed. Such critical thinking about the parameters most influential in controlling stress changes with pore pressure reduction has been used in numerous recent instances within Equinor in attempts to optimize field development decisions and build confidence in field development decisions. To demonstrate the value of this approach, examples are given where stress-dependent material parameters are implemented to match in situ stress testing results and the entire late-life workflow is applied to aid assets in critical development decision-making.
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