Carbonate Reservoir Facies and Multi-Pore System Analysis Using LWD Resistivity Imager Acquired During Pressurised Mud Cap Drilling - A Case Study Offshore Sarawak, Malaysia
Authors: Kyi Ko Ko , Dan Hui Xuan , Najm Ehab , Poh Chee Howe 
Published: September 2014

Facies and multipore system analyses are required for reservoir modeling as part of the field development plan for one of the largest non-associated gas bearing carbonate fields in Malaysia. To overcome the reservoir challenges, the wells in the field were drilled using the pressurized mud cap drilling (PMCD) technique, which significantly affected the type and quality of the necessary petrophysical data acquired for reservoir evaluation. This technique also added challenges when trying to standardize an evaluation technique for all wells in the field.

The well under study was logged using a resistivity imager tool while drilling with an exceptionally high image quality, which enabled the required analysis to be performed. Structural and stratigraphic analyses were performed using conventional techniques, and a correlation was made with the core analysis from a short cored section in the well.

The multipore system analysis was performed using a technique that transfers the image resistivity to a porosity map and breaks it into micro, mezo, and macro pore volumes by using one of several mathematical or statistical methods. The reservoir permeability was estimated by using a rock fabric technique that takes into consideration the contribution of inter-particle and secondary porosities.

Image-derived lithofacies, reservoir units from pore system analysis, and the electrofacies obtained from the quad-combo data acquired while drilling were used to characterize the reservoir and classify it into geologic and petrophysical units. A comparison between the various techniques was made, whereby similarities and discrepancies were analyzed to plan a standard workflow for the remaining the wells in this field.

This paper presents the difficulties encountered when using the data acquired under such a challenging drilling technique and the proposed workflow for reservoir characterization; it also uses a case study from offshore Sarawak, Malaysia to illustrate the advantages and disadvantages of the technologies and techniques used.

Introduction

The study well is vertical, 8.5-in. section drilled with seawater in pressured mud cap drilling (PMCD) mode with water-based mud (WBM) having a weight of 8.6 ppg. The well was drilled through a carbonate pinnacle, located offshore Sarawak in Malaysia, to further appraise the gas discovery made by the exploration well-1, which penetrated several hundred meters of gas column. Although the exploration well-1 was drilled without problems, the appraisal (study well-2) encountered severe losses, which resulted in the use of the PMCD technique to safely drill through the gas bearing carbonate reservoir. As a result, no attempt was made to conduct wireline logging because of well safety concerns. Therefore, it became critical to acquire as much data as possible, especially resistivity images, to achieve the operator’s set primary objectives. These objectives included the following:

Reducing volumetric uncertainties in terms of structure, porosity, and lateral reservoir distribution Distinguishing reservoir property distributions (porosity and permeability) Assessing presence and level of contaminants (Hg, H2S, CO2) toward the bottom of the gas column Testing productivity of the lower zones