Flow Unit Thickness and Permeability Evaluation in Horizontal Wells Using Logging While Drilling and Wireline Formation Tester Transient Data
Authors: Cig Koksal , Ayan Cosan , Mahruqy Sultan , Al-Shamsi Khalsa Abullah 
Published: March 2015

Oil was discovered in a carbonate reservoir which indicated discontinuous and complex geological features. The carbonate field in the Sultanate of Oman is at the early stages of development and reservoir uncertainties are still significant.

The uncertain geological features, proximity of possible faults and heterogeneous reservoir properties make horizontal well placement a difficult task. The latest logging while drilling (LWD) technology for well placement was utilized to track the distance to the upper flow unit of the reservoir. It was discovered that the formation was dipping upwards and had separate units. The lower flow unit boundary was uncertain due to the LWD distance detection limit. Flow unit thickness identification became an important task to understand the potential of the reservoir. The objective of this study in the complex reservoir is to present a new way to determine the flow unit thicknesses by utilizing the LWD and wireline formation tester (WFT) interval pressure transient tests (IPTT) data.

IPTT tests with a dual inflatable packer in combination with LWD logs resulted in local horizontal and vertical permeabilities and flow unit thicknesses along the horizontal well. Integration of the two distinct logging methods helped accomplish not only the well geometry and local petrophysical properties, but also gave information on large scale properties. The wells were also surveyed with open hole logging tools to obtain sedimentary features, to collect necessary oil samples and to obtain reservoir properties such as faults information, saturations, permeabilities and in-situ rock stresses. Following data acquisition and joint evaluation, an integrated study was conducted for the field development.

This paper presents an integrated solution of LWD logs and IPTT data evaluation. The approach had a considerable impact on field development plans and reserve calculations in the new and complex carbonate reservoir.

Introduction

Reservoir Geology

The geology of the reservoir in the study presents a stratigraphic trap in an Upper Shuhaiba carbonate formation sealed by a shale layer above and by argillaceous limestone facies laterally. The reservoir units represent the shoaling and upper slope portion of clinoforms that prograde into the basin. Rudist /stromatoporoid bioherm and reworked facies ranging from fine to coarse grained have been encountered in addition to more common orbitolinid packstones and wackstones. The quality of the reservoir represents a complex relationship between primary depositional facies and subsequent diagenisis in the form of both cementation and leaching (Fig. 1). Obtaining core samples subsequently assists understanding the complexity of the lithology. The thickness and permeability distribution of the reservoir is difficult to foresee without extensive logging and testing. Location and distribution of the reservoir quality facies represent one of the major uncertainties for developing of the reservoir. Formation microresistivity imager logs provide the fracture and facies distribution of the rock textures with the correlation of cores collected (Wang et al. 2008). Seismic data gives some indications of these structures and a recent reprocessing allows partial answers for reservoir units in some areas.