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№ 6 (June 2011)

Case Study. TOC Estimation Methodology in Accessing Shale Potential in India’s Cambay Basin

   Sprawling across the western part of India, the Cambay basin is one of the most petroliferous basins in India. After the discovery of oil in the landmark well Lunej-1, the basin has consistently added to the hydrocarbon supply of the country.

By Ratindra Nath Pandey, General Manager R&D, Gujarat State Petroleum Corporation Ltd. (GSPCL)

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   Approximately 5700 wells have been drilled by various operators in this basin with extremely high success ratio. GSPCL which has a Production Sharing Contract with the Government of India has kept Shale Gas as one of the targets while drilling these wells. After analysis of the systematic increase in the gas content, it was observed that C1 to C5 content became high as entry was made into the zones with higher TOC in Olpad and Cambay formation. This led GSPL to conclude that Cambay Shale has the potential to become an attractive target for tapping hydrocarbons.   

- Hydrocarbon exploration in the basin began more than fifty five years ago with eocene pay zones being the target of most operators, Oligocene and Miocene pay zones have been developed.

– Exploration conducted in entire sedimentary column as well as Basalts. The Ingoli Field has been producing oil from fractured basalts since 2004. Ongoing production from silt stones embedded areas.

– Geological setting of Ingoli field indicates a source rock in the western part, Olpad formation has TOC which generated hydrocarbons and could be an attractive target for harnessing hydrocarbon from shale.

– Tarapur-1 is the first well drilled in Tarapur tectonic block.

   The data obtained from drilling of wells for conventional hydrocarbons is used in carrying out an estimation of Total Organic Carbon (TOC) in different parts of Cambay basin. The available logs clearly show that TOC in these zones range from 2 to 9.5 percent which is quite good as per international standards. These estimates are byproduct of the conventional wells drilled for conventional traps. Hence it is imperative that once the wells are exclusively drilled for Shale gas, the chances of TOC percentage going up are quite high.

   The perforated interval demonstrates that even though the zone was perforated primarily for oil, it did yield gas also during the testing period. This is an indication that Cambay Shale does contain oil and gas which has the potential to become a target for production of gas.

Technology
Bearing in mind the aspects of environmental limitations of technology, GSPC while conducting stimulation job has been working on the development of eco-friendly methodologies. Bare foot testing of an interval within Cambay / Olpad section in a well, located in Ankleswar area of the basin was executed. The well yielded gas at the rate of 20,000 cubic meters per day (CMPD) with well head pressure of 3,000 psi in a vertical well. The well was drilled to 2,388 meters of depth and the portion below 1,752 meters was left open. The well remained closed for five months and 10 days and upon opening it has produced 15,000 to 20,000 cubic meters per day of rich gas.

   Since the barefoot section yielded rich gas from Lower Cambay Shale / Olpad section, a systematic test could not be carried out due to influx of sediments from the interval.
The source rock analysis of conventional core taken from this zone is presented in Table 1.

Technological Feats
Sonic imaging microscope creates an image of pore throats along with their connections with each other thereby depicting the permeability measurements in a quantifiable manner.

   Fourier Transform Infrared Spectroscopy enables accurate measurements of petro-physical parameters.

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