Efficient development of Sulige tight sandstone gas field

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The supergiant Sulige gas field, located in the Ordos Basin and featuring tight sandstone reservoirs of low permeability, low pressure and low abundance, is the largest in China with identified gas in place of 1,100.824 billion cubic meters.

With the daily gas production exceeding 10 million cubic meters in 2007, 20 million cubic meters in 2008, and 30 million cubic meters in 2009, Sulige had been able to produce 37 million cubic meters per day or 13.5 billion cubic meters of natural gas per annum by the end of 2010, becoming China's largest uncompartmentalized gas field, and a sample of low-cost development of tight gas reservoirs in the country. By September 30, 2011, there were accumulatively 4,222 gas wells in operation, and 3,439 of which being opened per day averagely. The supergiant gas field is expected to be capable of producing 4,600 cubic meters of natural gas per day at the end of this year.

Challenges

Sulige's toughness was recognized as early as in the preliminary development assessment:
· Well sites are difficult to be allocated because effective reservoirs are hard to be identified. · Cost-effective development is jammed by limited reserves controlled by individual wells and rapid pressure drops. · The new techniques and technologies used in the preliminary assessment and pilot tests do not perform as expected. · The extreme heterogeneity is different from any low-permeability gas field abroad or any others in China. This is not only reflected by the considerably varied reservoir distribution in the horizontal direction and multiple intervals with interlayers between each other in the vertical direction, but also by the much varied physical properties across reservoirs.
The following three major challenges posed by the tight reservoirs and the unique geological conditions in Sulige have to be tackled so that the gas field can be efficiently developed: · How to accurately predict gas reservoirs to increase drilling success rate. · How to enhance the output from individual well in tight sandstone reservoirs. · How to much reduce drilling costs and the investment in surface gathering works.

Technology and Innovation

To increase the production per individual well at a much reduced development cost, CNPC carried out technical research and field tests using the latest theories and leading technologies in geology, geophysics, gas reservoir engineering, gas production techniques, and surface gathering for the past decade from 2001 to 2010. Now we have addressed seven technical obstacles, namely, the prediction of thin gas reservoirs, productivity appraisal, development mode, rapid drilling, separate layer fracturing and commingled production, downhole choking, and inter-well concatenation for medium to low-pressure gas gathering. The resulted package of efficient development technologies for tight sandstone gas reservoirs has successfully tackled the world-class challenges and realized efficient development at a low cost.

Well-spacing technology aiming at improving the prediction precision of thin gas reservoirs
A well-spacing technology aiming at improving the prediction precision of thin gas reservoirs has come out of our independent research and development. To meet the distributing characteristics of the tight sandstone reservoirs in Sulige, a high-density full-digital seismic approach is used in place of conventional ones. The accuracy of reservoir prediction has been increased from 10-15 meters to 5-10 meters and the percentage of efficient wells from the initial 66.7% to more than 85% to optimize well site deployment in these highly heterogeneous reservoirs.

Productivity appraisal technology for tight gas reservoirs
A productivity appraisal technology for tight gas reservoirs that is leading in China has been independently innovated. This includes an improved productivity testing theory of fractured wells and an optimized testing method for low-yield wells. In addition, a first-of-its-kind chart for integrated interpretation of post-fracturing tests in tight gas reservoirs ensures quick and rational allocation of production in gas wells and much shortens the time to start a well. With the elimination of well testing, the time to start a well can be shortened by 7-10 days and the vent gas reduced by 80,000-100,000 cubic meters.

Development mode of supergiant tight sandstone gas reservoirs

China's first development mode of supergiant tight sandstone gas reservoirs has been created. The dynamic reserves controlled by each well are accurately appraised by uncovering the seepage mechanism of tight sandstone reservoirs under stress sensitive and retrograde condensation effects. A scientific production system and an efficient development well pattern are determined. This extends the period of stable production of gas wells and significantly increases the reserve-releasing rate. The final result is an efficient development approach featuring scaled application of clustered and horizontal wells.

Rapid drilling technology focusing on PDC bits

A rapid drilling technology focusing on PDC bits has been created. We developed a drilling bit with a cutting structure and anti-whirl design that is suitable to the tight sandstone in Sulige. This bit is part of a "four-in-one" drilling assembly which registered a new world speed record for drilling medium to deep wells in sand shale formations. These have much saved the drilling cost by enabling an average 18-day, as compared to the initial 38-day, drilling for an along-hole depth of 3,440 meters. In fact, the minimum drilling period can be as short as 7 days.

	Integrated technology of separate layer fracturing and commingled production for gas wells
	An integrated technology of separate layer fracturing and commingled production for gas wells has been developed. With our independent intellectual property rights, the technology use packers to effectively pack intervals to be stimulated. It consists of our independently developed low-damage guar-based fracturing fluid system and gel-breaking technology for separate layer fracturing, both of which are suitable to tight reservoirs and can minimize reservoir damage and improve settling profiles. In fact, the technology has made such a breakthrough that four layers can be continuously fractured in a gas well and thus doubles the production per well. The new technology of separate layer fracturing and commingled production with mechanical packers in gas wells that we have created and improved is the first of its type in China.

	Downhole choking technology with resistance against high temperature and pressure
	A world-leading downhole choking technology with resistance against high temperature and pressure has been independently developed. By using formation energy to realize choking and pressure drop along the borehole, conventional heaters at gathering stations or wellhead can be eliminated to suppress hydrate generation. It supports for the medium to low-pressure gathering and minimizes surface investment in Sulige field.

Inter-well concatenation for medium to low-pressure gas gathering technology

Inter-well concatenation for medium to low-pressure gas gathering technology has been independently innovated. It gave birth to China's third brand-new gas gathering mode, the first two being the "individual-well" and the "high-pressure" mode. Featuring "downhole choking and inter-well concatenation", the technology not only eliminates heating and methanol injection at wellhead, but also obviates the heat preservation on gas production pipelines. By embracing technical innovation and practice of "standard-based design and modular construction", it enables standard-setting surface construction in terms of the scale, speed, cost, and efficiency of gas fields. An innovatively integrated, solar-powered digital production management system with remote wellhead control and well data acquisition ensures safe, environmentally friendly, and efficient production of the gas field.

So far, more than 3,600 wells have benefited from the scaled application of this package of development technologies. The overall cost for each well dropped from RMB 13.17 million to RMB 7.51 million, resulting in accumulative investment saving of RMB 31.14 billion and profit of RMB 9.628 billion.