Use of Advanced Geomechanical Analysis to Define Practical Cut-Offs for Sand Control Decisions

Abstract

Abstract This paper demonstrates how a robust geomechanical sanding evaluation is used to develop pragmatic cut-offs and decision support charts for credible sand management and completion selections. Sanding (or solids production) has multiple adverse effects on well productivity and integrity and can pose health, safety and environmental hazards. Sanding has a geomechanical root and it can be managed: "avoided; mitigated, controlled or handled". An effective sand management strategy starts with the geomechanical characterisation of the target formations and sanding assessments. Once the field-calibrated geomechanical sand production prediction model has been established, using limited data such as a log-derived rock strength profile, decision flowcharts can be developed to assist well engineers for completion and sand control decisions by considering the well-life risk of sanding and borehole instabilities. Such decision charts enable well and production engineers to follow a quick, cost-effective and holistic approach for sanding assessment and completion selection. Two case studies are presented showing the development and use of practical cut-offs for real-time sanding assessments and completion selection. In both cases there existed calibrated geomechanical models. Case one is an offshore gas field with high rates of production from subsea wellheads and a long pipeline to an onshore plant. Production wells are 500-700 m long, nearly horizontal at 4000-4200m depth and with an expected pressure reduction from 6000 to 1000 psi. The reservoirs are strong, medium porosity sandstones in a highly stressed tectonic regime. Advanced geomechanical analyses, combining analytical and numerical methodologies indicated initial sand-free production, with sanding risk later in the field life from the high porosity zones. A fit-for-purpose near real-time workflow was developed for a selective perforation completion strategy combining petrophysics, geomechanics and cement bond log evaluation. Phase 1 development wells came online in early 2018 and to date all are producing sand-free with the expected gas flow. Case two is an offshore oilfield with multiple stacked sandstones and shale sequences at 1600-2200 m depth. Shallower reservoirs are depleting and deeper, mechanically stronger, less depleted layers are now the targets. In 2006, a sand production risk and life the well hole instability assessment was conducted for new horizontal laterals planned to be completed open hole (barefoot) with water production expected early in the life of the well. Using the calibrated sanding evaluation model, a simple flowchart was developed to assist completion engineers to make sand control decisions in real-time. All planned wells were completed open hole without sand control but did use pre-drilled liners to minimise hole instability risks in the shale layers. After nearly 17 years, all wells are still producing sand-free under high drawdowns and watercuts confirming the accuracy of earlier sanding predictions and well completion decisions. Pros and cons of pragmatic cut-offs are listed and the residual risks involved with such approaches are also highlighted.