Wireline Dual-Packer Formation Testing Operations in Basement: A Case Study from the West Coast Offshore, India
Authors: Singh Rajesh Kumar , Nangia Viraj , Dunlap James , Ghosh Arnab , Roy Dipanka Behari , Klimentos Theodore , Bhattacharya Shyamal , Sood Pradeep Kumar , Agrawal Ashwini Kumar 
Published: May 2011

Hydrocarbon production from fractured basements holds great promise as an unconventional hydrocarbon source. These unconventional reservoirs are key in meeting the world's increasing hydrocarbon demand. However, basement characterization faces great technical challenges. Traditional formation evaluation methods, such as saturation estimation using resistivity-dependent equations, are often ill-suited to the basement environment because of complex mineralogy including conductive and heavy minerals. A workflow integrating the results from various advanced well log interpretations provides a robust foundation for successful tapping of hydrocarbon potential from fractured basements.

In the case study presented, a workflow is described in which the fractured basement underlying the sedimentary rocks of an offshore development field west of India was tested for the first time using the wireline-conveyed dual packer interval pressure transient testing (IPTT) technique. Pressure-volume-temperature quality oil samples were collected, gas/oil ratio was measured in real time, and formation pressure was recorded. The test was made possible by effective screening of potential test intervals using an enhanced workflow in which the newest generation of the nuclear magnetic resonance scanner was used for hydrocarbon saturation and permeability determination in conjunction with classical openhole logs and other advanced logs. Stoneley waveform analysis and borehole micro imager-derived results were useful in detailed fracture characterization and optimization of the dual packer position for a successful IPTT operation that proved oil production from fractured basement.

Introduction

Due to decline in the production rates of the field and increasing energy demand, there has been an intensified quest to explore hydrocarbons. The field, located western offshore India, is producing from a carbonate reservoir, there exist a fractured basement beneath this carbonate reservoir that might be hydrocarbon bearing. If properly characterized and evaluated, this basement could open up a new source of hydrocarbons that could be boon to India's oil industry. Wells were drilled deeper into the fractured basement, and the first few wells were evaluated with conventional openhole logs. The oil potential of this fractured basement was learned when the wells flowed oil to the surface during barefoot drillstem testing, revealing a new source of oil. Further testing showed that the new found oil reservoir was not as simple to characterize or evaluate as new wells produced oil at different rates during production testing. Figure 1 shows basic openhole logs of two different wells in which one well tested at a rate of 800 bbl/d and the other at a rate of only 40 bbl/d. It is difficult to ascertain the reason for this difference in flow rate on the basis of a basic openhole log shown in Figure.1. This variation in rate hints at a complexity of fractures and formation that needs to be understood before the basement can be exploited. Variation could be in terms of fracture orientation, or possibly only a particular type of fracture set might be hydrocarbon bearing, or there could be a well placement issue in this fractured basement. These variations could not be addressed by running conventional logs only.

This paper describes what can be done to better understand and characterize the fractured basement through an integrated approach, incorporating elemental capture spectroscopy, dipole sonic, and formation microresistivity image data to characterize the complex architecture of the rock. Elemental capture spectroscopy provided dry weight elemental composition while microresistivity image data provided morphological and textural information in addition to resistivity variation. Elemental capture spectroscopy data and image data, when integrated, bring out the compositional variation along with textural details, with fracture and foliation patterns. The information obtained from these logs can be decided to isolate the desired fractures to enable Interval Pressure Transient Testing (IPTT) by using dual packer, which would give us formation pressure and fracture properties such as storativity and interporosity flow parameters.