Approaches in Evaluating Low-Resistive Formations
Authors: Palar S. , Sutiyono S. 
Published: December 2007

Oil and gas fields in offshore East Kalimantan, Indonesia have a characteristic of having stacked hydrocarbon pays. Most of oil and gas are produced from Miocene-age Mahakam Delta sediments. Numerous oil and gas reservoirs from several depositional sequences demonstrate low-resistive pay characteristics. The low resistivity can be attributed to the presence of conductive water, clays or the effect of thin bed.

This paper presents two approaches, to be run in addition to the conventional wireline logs, in order to help in concluding the questionable pays. Employing one or both approaches can vastly improve the interpretation of low resistive formations. Both methods have been tested and tried in more than 30 wells, with the majority of the wells used only the new wireline formation tester tool.

Utilizing new measurements from nuclear magnetic resonance tool is the first approach used in the evaluation of low-resistive reservoirs. The tool can provide the information about the amount of irreducible water and movable fluid. The data are then combined with log interpretation from conventional wireline data to determine the amount of hydrocarbon more accurately, and to find more producible oil and gas behind the pipe. With this data, a relationship between low resistivity and irreducible water saturation can be possibly made.

The second approach is the use of a new wireline conveyed formation tester tool. This new tool can identify formation fluid downhole. It eliminates the necessity to bring fluid samples to the surface for hydrocarbon confirmation, therefore the formation evaluation time is greatly reduced. Presence of oil, water or gas - interpreted from the resistivity reading, temperature and infrared spectrometry - is monitored in real time at the surface.

Introduction

Unocal Indonesia Co. is operating several offshore oil and gas fields (i.e. Attaka, Sepinggan) in East Kalimantan, Indonesia. A typical characteristic of the fields is the presence of numerous sandstone reservoirs from 2,000 feet to 10,000 feet subsea depth. The reservoir thickness ranges from several inches to more than 100 feet. It's not uncommon to have a well with tens of reservoirs with a combined hydrocarbon pay of more than 100 feet.

Evaluating all reservoirs during logging usually do not resolve the fluid saturation of all of them. Wireline logs provide a starting point in the formation evaluation. However, the formation evaluation is complicated by the presence of numerous low-resistive reservoirs, among other things. Getting surface sample of all the questionable pays is not practical nor economical considering the expensive rig time and the logging cost.

The objective of the approaches is to minimize the bypass of low-resistive pay zones and to maximize the recovery of oil and gas from the low-resistive formations.

Geology of East Kalimantan Fields

Unocal's fields in East Kalimantan, Indonesia are located within the Kutei Basin which is characterized by the Samarinda Anticlinorium, a series of northeast-southwest trending anticlines. The basin is surrounded by Kuching High in the north, a rift with North Makasar Basin in the east, Adang Fault as southern boundary with Meratus Uplift and Paternoster Platform and Barito Shelf in the west. Major hydrocarbon traps are generally controlled by the structure, closures and faults. The productive reservoirs are mainly sandstone of Tertiary sediments of Middle Miocene to Pliocene age.

Oil and gas have been produced primarily from the deltaic sands which are normally thick, clean, blocky and highly resistive. P. 173^