Thistle Field Development
Authors: Nadir F.T. 
Published: December 2007

Nadir, F.T.;* SPE, BNOC (Development) Ltd.

Summary
A mathematical reservoir model of the Thistle field was used to explain the rapid pressure decline in some early development wells. This paper describes (1) how evidence was gathered to confirm the hypothesis that the Thistle reservoir is divided into three fault blocks and (2) the changes made to the development plan to arrest and reverse the field's pressure and plan to arrest and reverse the field's pressure and productivity decline. productivity decline.

Introduction
The Thistle field is located about 125 miles (201 km) northeast of the Shetland Islands (Fig. 1) in the very prolific northern part of the Viking graben. The prolific northern part of the Viking graben. The field, discovered in July 1973 with the completion of Well 211/18-2 is mainly in U.K. Offshore Block 211/18 but extends into Block 211/19. The participants in Block 211/18 are the British Natl. Oil participants in Block 211/18 are the British Natl. Oil Corp. (BNOC), Deminex, Santa Fe, Tricentrol, Burmah Oil, Charterhouse, and Ultramar. Block 211/19 participants are Conoco, Gulf, and BNOC. Blocks 211/18 and 211/19 were awarded, respectively, in the third and fourth rounds of U.K. offshore licenses. BNOC (Development) is operator for the unitized field.Nadir and Hay presented a comprehensive history of Thistle field in which they discussed the initial development plan and its objectives and compared the reservoir thickness and quality found in the first seven development wells with what was predicted. This paper updates the earlier work by predicted. This paper updates the earlier work by emphasizing reservoir engineering studies conducted to pinpoint the reason for the rapid pressure decline and, hence, production decline. It describes modifications to the development plan that allowed water injection wells to be located correctly to repressurize the reservoir and reverse the trend of declining production.

Geology
The Thistle Field is an easterly dipping Middle Jurassic Brent sandstone reservoir (Fig. 2) with an average net pay thickness of 380 ft (115.8 m). The formation is subdivided into four reservoir sands: D, C, B, and A. Stratigraphy and the reservoir quality were described in detail by Nadir and Hay and updated by Hallett. There have been few changes to the original geological model; the main change is some evidence of erosion between the Sands D and C, less Sand D erosion, and the possibility of syndepositional fault movement. The quality of the sand is slightly better than originally expected.

Reservoir Modeling
Production from Thistle field started in Feb. 1978, Production from Thistle field started in Feb. 1978, and the eighth development well was completed in Aug. 1978 (Fig. 3). The wellhead pressures of the producing wells in the crestal area of the field were producing wells in the crestal area of the field were declining faster than anticipated, which could have been a result of localized pressure sinks and/or a small or nonexistent aquifer. The completion of Well 08A showed Sand-B pressure to have been depleted by 1,200 psig (8274 kPa), which dispelled any localized pressure sink concepts. The search for a solution started.A black-oil mathematical reservoir model was used to match field performance. The matching parameter was pressure. Wellhead flowing pressures, measured with dead-weight gauges, were converted to bottomhole flowing pressures by Orkiszewski's method and to equivalent grid-block pressures by Peaceman's method. Bottomhole pressure buildup Peaceman's method. Bottomhole pressure buildup surveys were used as spot checks to confirm the reliability of the resultant grid-block pressure values. Repeat formation tester (RFT) pressures identified the different pressure regimes in the various sands at a specific point in time at some distance from producing wells. producing wells. JPT
P. 1828