The Miller field lies in blocks 16/17b and 16/8b of the UKCS and was discovered in 1982. The Miller reservoir is an Upper Jurassic submarine-fan sandstone with a typical true vertical depth of 13,200 feet (4000m). The field currently produces circa 125mbpd from seven wells. The oil contains 11% by weight of carbon dioxide and the gas typically contains 200 ppm (vol/vol) of hydrogensulphide. General reservoir fluid parameters are listed in Table 1.
The development plan for the field (the Annex 'B') was approved by the UK government in 1988. In the development plan it was intended to pre-drill10 wells from a template prior to platform installation. Development drilling from the template commenced in 1989, but due to severe drilling problems only 7 wells (four producers and three injectors) were drilled prior to platform installation in the summer of 1991. These template wells were tied back to the platform and brought on stream in June 1992. Platform drilling commenced in August 1992. By November 1993 three additional production wells and two additional injection wells had been drilled from the platform. Development drilling will continue through to 1997, with a total of 27wells (18 producers and 9 injectors) planned.
Due to the high production rates and the low number of wells, it is important to minimise formation damage as this will both maximise the time on plateau and reduce any stimulation requirements (and hence associated downtime).This paper focuses on the significant and complex formation damage problem seen on Miller to date, the steps taken to reduce this problem, the skin factors seen on the seven development production wells drilled to date and the strategy for damage minimisation in the future.
It was clear from the appraisal stage that Miller wells were highly sensitive to formation damage as the average skin factor for the 12 exploration/appraisal wells was +5. Skins as high as +185 were recorded in some tests. A complete review of field appraisal well data was conducted in 1986. This review covered all areas which could have impact on well productivity, including sedimentology, drilling/testing practices and formation fluids. Damage was considered to be unlikely to be due to sedimentology (no significant swelling clays or mobile fines). Cementing practices were also ruled out, using tests which indicated maximum depth of fluid loss during cementing was less than 1.1" (a depth easily penetrated by the perforating guns).
A typical Miller formation water analysis as derived from the appraisal wells and a typical North Sea water analysis are presented in Table 2.The produced formation water has a high potential for calcium carbonate scale deposition.