Applications Of 3D Geomechanical Model On Sand Production Prediction And Completion Design In Zawtika Gas Field

Abstract

In-situ stress state of the reservoir rock is generally changed upon well drilling, well completion and production or injection. To some extent, the rock will fail if the resulting stress around well bore exceeds rock strength. As a result, it could create drilling difficulty, hole collapse and sand production. This is, however, not commonly seen in typical reservoir where rock strength is normally high, but not for Zawtika field. In Zawtika gas development project, sand production problem has been encountered due to its nature of weak reservoir rock. Generally speaking the reservoir UCS ranges between 500 to 1500 psi which is extremely low compared to typical sandstone. The consequence of this obviously showed up during exploration and appraisal phase where the well testing (TST) showed sand production in most reservoirs, especially in shallow ones. Stratigraphically, gas reservoirs in Zawtika field lie between H-05 and H-50 marker whereby the shallow reservoir associates to higher risk of sand production than deep reservoir due to the fact that it has lower rock strength based on core-tested data. Rock tends to get stronger with depth likely due to better compaction and cementation as a result of longer deposition. It is foreseen that severe sand production will be unavoidable upon development phase if there is no mitigation plan. To effectively mitigate sand production problem, 3D Reservoir Geomechanics model (3D MEM) was utilized to identify, predict and mitigate sand production problems on this gas development project.

In this study, sand production prediction analyses were undertaken using commercial software equipped with sand prediction module (Schlumberger’s Techlog Completion Geomechanics). The model accounts for scale effects associated with different perforation diameter, borehole and sand grain diameters and plasticity effects. Additionally, orientation of the cavity (wellbore or perforations) and production conditions can have a significant impact on sand production risks. A reservoir may produce sand-free from cavities in certain directions, but not in other directions, due to the difference in the stress acting on the cavities. Furthermore, a reservoir may be produced sand-free at the beginning, but sand production may occur at a later stage due to reservoir pressure depletion and pressure drawdown applied to produce. We also determine the critical drawdown pressure (CDDP) for both open-hole and cased-hole perforated completions, and evaluate the effects of depletion on the stability of the cavity and its sanding propensity for the life of the wells. Similar to reservoir simulation history matching, the sand production prediction model is validated with actual TSTs and sand production test results prior to prediction. On this matter a down-hole sand scanner tool along with production logging were run in our well to acquire actual sand production, FBHP and CDDP data for model validation. Another crucial parameter is diameter of perforation tunnel. This combined with grain diameter has significant impact on sand prediction results. To capture it correctly, we also predict perforation diameter based on UCS and stress extracted from 3D MEM.

In summary, this paper elaborates how 3D Reservoir Geomechanics model can be used to optimize well completion design for future Well Head Platform(s) which consist of both Cased hole gravel pack (CHGP) and Cased hole perforated completion. Furthermore, the model is used to evaluate other alternative completion such as open-hole gravel pack (OHGP) completion and vertical open-hole completion (VOC). This is to seek other completion options that can be potentially applied for gas field development from weak reservoirs with lower cost than existing CHGP completion.

AAPG Datapages/Search and Discovery Article #90324 © 2018 AAPG Asia Pacific Region GTW, Pore Pressure & Geomechanics: From Exploration to Abandonment, Perth, Australia, June 6-7, 2018