Abstract

This paper reviews the development and results of clean-up acidisations in the Leman Gas Field in the Southern North Sea.

Leman is a large, dry gas field producing from the massive Permain sandstones, the Rotliegendes, which are found between the Carboniferous coal measures and the Zechstein evaporites. Although this low permeability quartzitic sandstone would normally be considered to be a classic fracturing candidate, acidisation has been found to be the most effective stimulation method. Rather surprisingly, weak (7 1/2 per cent.) hydrochloric acid has been found to be just as effective as Mud Acid in terms of improvements in productivity.

To date, Shell/Esso have carried out 28 acidisations on producing wells and 10 limited entry acid jobs on new completions. The resulting increases in potential varied from zero to 234 per cent., with an average of 40 per cent. being attained on the older wells, while on the new completions about 70 per cent. was obtained.

The paper discusses the selection of stimulation candidates; the treatment design; and the effects on the reservoir rock, inflow profile, and interpreted reservoir parameters. As the wells are extensively perforated (up to parameters. As the wells are extensively perforated (up to 1,800 holes) over a thick reservoir section (up to 700 ft.) of highly inhomogeneous rock, diversion was a major problem. A number of techniques are reviewed and problem. A number of techniques are reviewed and experiments with several novel methods are reported, Atomized acidisations were also tried and appear to be attractive under suitable field conditions. Some of the problems that were experienced are briefly discussed and problems that were experienced are briefly discussed and the paper concludes with recommendations for production engineers embarking on a major stimulation campaign.

Introduction

The Leman Gas Field in the southern North Sea was discovered by Shell/Esso in April, 1966. The large, faulted, anticlinal structure is located within blocks 49/26, 49/27, 49/28 and 53/2. The gas is produced from a massive Permain sandstone, the Rotliegendes, which is found between the Carboniferous coal measures and the Zechstein evaporites.

The gas is sulphur-free and contains very little carbon dioxide (0.04 per cent.), the main constituent being methane (95 per cent.). Under reservoir conditions the production is completely dry; however, 50 deg. API condensate production is completely dry; however, 50 deg. API condensate drops out within the production string. Before the gas is sold, approximately 1 barrel of condensate is recovered per million standard cu. ft., of which less than half is per million standard cu. ft., of which less than half is formed within the well bore (this proportion will increase as compression is introduced). A small quantity of water (0.07 bbls/MMscf) also condenses from the vapour phase as the gas is produced. In general, the liquids are satifactorily transported from the wellbore, although liquid hold-up is observed with the poorer wells (less than 5 MMscf/d).

The porosity of the reservoir rock is generally low with an average value of about 14 per cent. (range 5–25 per cent.). The formation is extremely heterogeneous so that permeability, although roughly related to porosity, is one of the most uncertain reservoir parameters. Moreover, production logs have shown very little correlation between production logs have shown very little correlation between porosity and deliverability. porosity and deliverability. The field has not been fully unitised and is being developed separately by two operators, Shell (for Shell/ Esso Group, Block 49/26) and Amoco (for the East Leman Unit, Blocks 49/27, 49/28 and 53/2). Both groups have a fixed sales contract with the British Gas Corporation, and are allocated production according to the current Seller's Proportions.

Shell/Esso have developed their section of the field by drilling deviated wells from fixed platforms. These have generally been located at the crest of the structure. To minimise impairment, invert oil emulsion mud (IOEM) was used to drill the gas zones.

The wells have mostly been completed with a 7-5/8 in. production casing, although many have 5 in. liners across production casing, although many have 5 in. liners across the pay. The completion consists of a permanent production packer set just above the reservoir and a predominantly packer set just above the reservoir and a predominantly 5 1/2 in. production tubing string. High deliverabilities were necessary during the early life of the field, so that the wells were heavily perforated at the time of completion, with 4 shots/ft. being made in all intervals with a porosity of more than 14 per cent. (up to 300 ft. in some wells). Although the lower permeability sections were generally shot first, they were allowed very little clean-up before the better zones were brought on. Perforating under drawdown was not introduced until about 60 per cent. of the wells had been completed.

ROCK PROPERTIES

The general lithological description of the Leman reservoir rock is as follows: sandstone, red/brown (occasionally buff to grey/green); predominantly fine grained but ranging from very fine sand lower to medium sand upper with some very coarse stringers; generally well sorted; sub-rounded to sub-angular grains; friable to moderately hard; argillaceous in parts, with occasional carbonaceous and anhydrite inclusions.

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