Objectives/Scope: When drilling a micro fractured formation, the fluid differential pressure for wellbore support can be lost and the unsupported formation may fail, causing losses, pack-off and instability. This may be minimized or resolved if the zones of pre-existing fractures are known and the trajectory is optimized to avoid them. In the Malay Basin, one structure of interest has undergone several periods of stress and faulting, at times, strike-slip in nature. Wrench faults in the area are causing compression and extension. This strain causes micro fractures and voids to be generated. The formation has a secondary porosity but seismic resolution does not allow the pre-existing fractures to be seen. By using a technique known as Geomechanical Forward Modelling (GFM), it is possible to determine natural fracture distribution and avoid these zones, thereby decreasing the risk of encountering instability caused by secondary porosity. The problems of borehole instability encountered in the area are demonstrated in wells, A1, A2 and A3. The most problematic well was A3. It had many incidents of pack off causing side tracks. In each case, the hole condition was better on side-tracks made higher up the well. It was considered that there is a 3 dimensional extent to the presence of micro fractures. The fracture intensity may have a vertical as well as areal extent. Sometimes wells crossing above or below a zone are not as difficult as through it. On wells that experienced pack off, cavings at the shaker had an abundance of linear breaks oblique to shale bedding surfaces and suggests the probability of pre-existing fractures. Pre-existing fractures were also seen on image logs. During planning for an appraisal well, zones with lower plastic shear strain were identified to the North and West of the planned well trajectory by applying GFM. The plastic shear strain profile along the planned trajectory suggested that the target depth will intersect a potential fractured zone. By moving the surface location by only 500 meters, the chances of encountering pre-existing fractures was significantly diminished. By applying Geomechanical Forward Modelling, (GFM), it is possible to determine distribution and possible locations of secondary fractures, too small to be determined from seismic. This allows for the selection of well trajectories to avoid these naturally fractured zones, thereby decreasing the risk of encountering instability from secondary porosity (pre-existing shear fractures).
Previous production drilling in the Bekok field has demonstrated two major challenges to a successful campaign. Firstly, substantial borehole instability can be encountered when drilling deviated wells through the Terengganu shale (K shale). The K shale was previously thought to be a reactive shale and that the problem originated from fluid-shale interactions. Secondly, invert emulsion oil based mud had previously been utilised on earlier production drilling but substantial productivity impairment was experienced against the K sands. A study of three synthetic based mud systems was carried out in return permeability tests on K sand cores. One emulsifier system was identified as demonstrating significantly less impairment. This system when applied, allowed for good productivity in the K reservoir. Exploration wells drilled with potassium chloride polymer muds showed low potassium depletion levels, suggesting that the shales were not reactive. XRD/XRF analysis of side wall cores from offset fields indicated that the shales consisted dominantly of small surface area clays, Table 1. This mineralogy and the low salinities of the shales suggested that the shales might be relatively weak. A study of the Malay Basin shales using extended leak off tests, electric log data and tests on shale core provided a geomechanics model that gave minimal wellbore instability. Field experience further defined the model. The application of required mud weight support to balance induced stress resulted in greatly enhanced wellbore stability. Bekok B-12 well was the first well in the Bekok field revisit to pass through the K shale to penetrate the K reservoir and as such is a test well for the studies made prior to drilling operations. Introduction Bekok field is located offshore in block PM-9, about 110 km east of Kerteh, Terengganu. The field was discovered in 1971 by Esso Production Malaysia Inc. (EPMI) and a total of 6 exploration wells were drilled to delineate the field. These vertical wells were drilled with no significant shale instability. Field development started with the installation of Bekok-A in 1978, Bekok-B in 1979 and Bekok-C in 1981. The field operatorship was handed/transferred to Petronas Carigali Shd. Bhd. (PCSB) in 1995 while EPMI still maintaining some equity.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
Copyright © 2025 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.