document

Abstract

Bombay high, an offshore oil field was discovered in the year 1974 and was put on production in May1976. Water injection in this field was started in 1984. The main pay zone L-III is of miocene age. L-III is a multilayered, anisotropic limestone reservoir with a gas cap and partial water drive. L-III reservoir of this has 13 different sub layers, which are separated by shale streaks.

Water Shut off jobs: Bombay high wells started producing water since 1984-85. Preferential depletion of high permeability layers in a set of high and low permeability layers perforated together and subsequent preferential flooding of these layers has led to an early break through of water as well as water loading of wells. This has also led to the leaving oil un-drained in less permeable layers. Earlier attempts to shut off water with conventional squeeze jobs met with a little success. However candidate selection with proper diagnostics and mechanical isolation of the squeezed layer has resulted in remarkable success. But this could not address to the middle water producing zones. In order to overcome the limitations of conventional cement squeeze jobs and in search of better success ratio on sustained basis and with the advent of various gel-chemicals, a number of gels of reputed companies were used during 1993-97 for water shut off jobs. However the results were not encouraging either. Recently during 1999-2000, extensive lab studies were done on cores simulated at reservoir conditions and the compatible gels were used. Three water shut off jobs have been done using these gels.

Gas shut off jobs: Success ratio remained low with the cement squeeze in between the oil producing and gas bearing layers. This increased remarkably with improved diagnosis by multidisciplinary team, use of clean work over fluid and mechanical isolation of the squeeze point with packer. Recently during 1999-2000, various gels were tested for compatibility with reservoir rock and fluid and accordingly the compatible gels were used in combination with cement cap for Gas shutoff jobs in 6 wells. Test results of 4 wells have shown that two of the four wells have been successful. With improved placement technique the jobs are expected to be more successful.

Water Control jobs

Description of formation in general.

In order to appreciate and comprehend the attempt of Water and Gas control jobs, it is quite relevant to understand the structural setting of the main pay zones and the stratigarphic sequence of the pay zones. Bombay High structure extending over an area of about 1500 sq. kms is a mega regional uplift created by doubly plunging anticline towards North and South with a sharply faulted eastern link and a gentle Western homocline.

The Bombay High field is separated by a graben area into Bombay High North and Bombay High South. Bombay High North is basically the western flank of the anticline trending SE-NW direction dipping gently towards North West. Bombay High South is western flank of the anticline dipping towards west and South. From the structural setting of the pay zone it can be easily seen that the gas cap area envelops the South-East part of the Bombay High North field. In Bombay High South field, gas cap covers the North-East part of the field. Both the fields are supported by partial water drive from western side extending from north to south. In order to provide pressure maintenance in the field, peripheral water-injection was started. However the benefit accrued on by the peripheral water injection was insignificant. Therefore two rows of water injection were installed in the central part of the field.

The problem of high water cut

The main pay zone L-III is multi-layered, anisotropic lime stone reservoir with a gas cap and partial water drive. L-III reservoir has 13 different sub layers viz. A1, A2-I, A2-II, A2-III, A2-IV, A2-V, A2-VI, A2-VII, N, B, C, D, E, which are mostly separated by shale streaks. A typical log of L-III layer shows various layers with a wide range of porosities.