TIGHT GAS RESERVOIRS - AN UNCONVENTIONAL NATURAL ENERGY SOURCE

SAAD ALI GHOURI, MOHAMMAD DANISH HASHMAT
Aug 03 - 09, 2009

Pakistan is a land blessed with many natural resources. Of which natural gas is a major and most consumed natural resource. Natural gas reservoirs can be classified into two types: conventional and unconventional. Conventional reservoir being the one that can be produced at economic flow rates and will produce economic volumes of gas without the use of any special recovery techniques almost all producing gas fields in Pakistan are producing from conventional reservoirs.

While unconventional reservoirs cannot be produced at economic flow rates using conventional methods. They require advance recovery processes and technologies such as hydraulic fracturing, acid fracturing, multi-lateral drilling etc. Typical unconventional reservoirs are tight-gas sands, coal-bed methane, heavy oil, and gas shales.

Tight gas sand reservoirs are a category of unconventional gas resources. Technically speaking they are gas bearing sandstone reservoirs having permeabilities less than 0.1 mD i.e. the interconnection of pores in the gas bearing reservoir is very less. Thereby these reservoirs could only be produced under advance technologies that are costlier than conventional techniques. Hence tight gas sands are mostly producible only under improved gas price and/or tax conditions.

More than 33Trillion cubic feet (25,700 MMBOE) gas initial in place have been estimated in Pakistan, while large area is still unknown, these regions are distributed in the Middle Indus Basin, Sulaiman foldbelt and Kirthar foldbelt. Tight gas in Potwar, lower Indus Basin and offshore region is yet to be estimated.

In Pakistan, almost every industry is dependent on gas the power sector being the major consumer. Moreover it is widely used domestically as evident from the 2001-2002 Economic Survey of Pakistan.

% CONSUMPTION OF GAS BY SECTORS, PAKISTAN (1990-91 TO 2000-01 AVERAGE)

Most of the conventional reserves of Pakistan are nearing depletion. Pakistan thus needs to sustain the supply of gas because there are no alternatives yet. Though alternate energy resources should be looked into but the existing potential must also be explored and made use of.

As the reserves are depleting with the passage of time and with the growing increase in natural gas consumption both in the commercial and public sector, as evident from the increase in CNG activity statistics for the financial years of 2005-06 and 2006-07, the large volume and long-term potential, attractive gas prices and unprecedented interest in world markets bring this unconventional gas resource into the forefront of our energy future.

SERIAL NO. CNG ACTIVITY BY THE END OF 2005-06 BY THE END OF 2006-07 PERCENTAGE INCREASE
1. Total operational CNG stations, till end of financial year. 1003 1488 48%
2. Vehicles converted to CNG. 970000 1080000 11%
3. Total number of provisional licenses issued till end of financial year. 3997 6215 55%

According to the "Economic Survey 2007-08 during the last ten years consumption of gas has increased at an average rate of 7.6% per annum. This is still following an increasing trend. The supply and demand shift has made it feasible and economical to produce these unconventional reservoirs. Thereby Pakistan has a market as well as potential to produce gas from tight gas reservoirs.

In our undergraduate project, under the guidance and support of Eni Pakistan Limited, different options in tight gas wells have been evaluated. Tight gas reservoirs require expensive technologies like hydraulic fractured vertical and horizontal wells for development and optimum gas recovery. To make investments confident very rigorous well/reservoir modeling is required, taking care of all uncertainties. Long-term changes in well/reservoir behavior are also modeled as long wells life is expected in tight sand environment.

The option of different wells: vertical hydraulic fracture, horizontal multi fractured wells have been tested both with analytical and simulation (Eclipse* environment) approach. Comparisons between different options have been done based on reserves and initial production.

The results showed productivity performance enhancement under stimulation conditions, i.e. the reservoir which could not produce economically under unstimulated conditions produced economically when hydraulic fractured, comparison with multifractured horizontal well showed further enhancement. This was done with due consideration to risk analysis for main uncertainty parameters.

It signifies that tight gas sand reservoirs can be feasible to produce. The government and the industry need to focus on this hidden potential of Pakistan as we now have the required technology and economic scenario.

*Eclipse is reservoir simulation software used of reservoir modeling