Improved Understanding of Static and Dynamic Geomechanical Stability and BHI data Integration: Key for Improving the Completion of a Horizontal Well in Tiered Complex of Laminated Unconventional and Fractured Carbonate Reservoirs.

Abstract

The deep, sub-salt reservoir complex is tiered with fractured tight carbonate at bottom and top, with the two sub-units of "upper unconventional kerogen?? and "lower inter-bedded kerogen-carbonate?? in the middle. This depositional setting is challenging for horizontal well placement where the thicknesses of respective sub-units are about 50 and 30 feet with varying geomechanical and petrophysical properties. Additionally, this complexity poses limitations in completions and effective stimulation of the Kimmeridgian-Oxfordian reservoirs in several gas fields at development stage in Kuwait.

A horizontal well is placed in the lower sub-unit of the laminated complex of unconventional kerogen and fractured carbonate reservoir as a Maximum Reservoir Contact (MRC) type well. A pilot mother-bore was drilled and logged to identify the lithological properties across the entire vertical domain - facilitates the optimization of horizontal drain-hole placement within the targeted reservoir units.

No wellbore stability issues in drilling were predicted based on the geomechanical understanding where core-calibrated logs from offset vertical wells were considered. However, this modeling method did not have the functionality to integrate the impact of drawdown on the laminated formation which became unstable and collapsed during the short open-hole drill-stem test (DST) plugging the tubing prior to the final completions. An alternative "book-shelf?? geomechanical model was considered at pre-drill stage for predicting the wellbore stability. Once the drilling was completed, the time-lapsed multi-arm caliper indicated the validity of the alternative methodology in predicting the unstable stack of laminations in kerogen-rich strata.

The paper discusses an optimization methodology to enhance the understanding of static and dynamic geomechanical stability through the use of BHI data. Objective of the proposed method is to help improve the effectiveness of completions where wellbore stability due to geomechanical complexity in stacked-pay reservoirs is a primary wellbore challenge in deploying the completions and executing a subsequent stimulation and testing campaign.

Introduction

The paper discusses the issues encountered in the deep sub-salt horizontal well completion and limitations of conventional mechanical earth model (MEM) application, which has successfully addressed the challenges during drilling. The stability of rock under different loading conditions in the subsurface environment during drilling and production is controlled by the strength of the formation. These properties are therefore basic and essential parameters for any geomechanical analysis and prediction. The rock mechanical properties include elastic constants (Young's modulus and Poisson's ratio), and rock strength data such as unconfined compressive strength (UCS) and internal friction angle (f). Log data, notably compressional and shear slowness and rock bulk density, are utilized to compute rock mechanical properties trends along depth in a given formation interval. However, as log-based mechanical properties are dynamic in nature, core-based properties from laboratory measurements are used to calibrate the log-based properties and generate their static equivalents. Additionally, UCS and friction angle values cannot be directly inferred from logs without using some correlations between measured values (from cores) and log parameters (Plumb, 1994). The continuous profiles of calibrated rock properties give important indication of natural variations of formation competence and the presence of relatively weak and strong layers along depth for use in drilling and completions optimization. The MEM concept and its application to high-risk well construction projects are key to achieve success in drilling and completing the technically and economically challenging wells on budget (Plumb, 2000).