Stimulation in the Tommeliten Field

Abstract

This paper describes the planning, design, execution and results of massive fracture acidizing treatments in the Tommeliten Field, offshore Norway. The Tommeliten is a marginal field consisting of subsea completions. Difficulties in effectively stimulating the Tommeliten wells arise from numerous factors including the relatively soft and homogeneous nature of the chalk reservoir, large intervals requiring staged treatments, high treating pressures, high formation temperatures, and the logistics of performing the treatments via a semi-submersible drilling rig and stimulation vessel. Based on experience in other chalk formations, a general treatment program was formulated. Treatment design utilized multi-stages of viscous pad followed by acid and over flush. Fracture geometry was simulated by means of computer design programs. Extensive laboratory testing was then carried out to provide data for improving the program. As each well was treated the program was modified based upon information from pre-fracturing injection tests and the main treatment profiles. One and one-half years production data were available at the writing of this paper and form the basis for the productivity evaluation. INTRODUCTION The Tommeliten Field is situated in block 1/9 in the western part of the Ekofisk Area of the North Sea (Fig. 1). This field is operated by Statoil with Norske Fina and Norsk AGIP as partners. The field consists of two distinct and separate structures, Alpha and Gamma. Phase I of the Tommeliten project consisted of six wells in the Gamma structure. This project, a subsea development in water depth of 250 ft, is based on the use of existing infrastructure of the Ekofisk complexes. The six Gamma wells have been drilled through a single subsea template and are tied to the Edda platform, 7.5 miles to the northeast, via a subsea manifold. Drilling of the Tommeliten wells was described by Sunde.1 The subsea template has been described by Solheim.2 Completion and stimulation of these wells took place during the months of May-August 1988 and contractual gas deliveries commenced in early October 1988. Reserves in the Tommeliten Field are found in the Ekofisk and Tor formations, chalk sediments of the Tertiary and Upper Cretaceous Period (Fig. 2). The reservoir is a massive chalky limestone with high porosity and low matrix permeability, similar to other reservoirs in the Ekofisk Area. Combined thickness of the Ekofisk and Tor intervals ranges from 300 to 700 ft. Pressure and temperature at a depth of 10,168 ft mean sea level (MSL) are approximately 7,050 psi and 265°F.3 The gas/oil ratio was expected to be about 1,470 Sm3 /m3.4 Statoil discovered the Tommeliten field in 1976, but its relatively small size and a lack of realistic commercial outlet for the gas delayed development. One significant benefit of the long lead time between discovery and production is the extensive experience which has been gained regarding stimulation techniques for North Sea Chalks and specifically for Ekofisk Area wells. REVIEW OF STIMULATION TECHNIQUES Chalk reservoirs are an exploration objective in many parts of the world. Chalks typically have high porosity, however, due to their low permeability they are only marginally profitable unless they can be effectively stimulated.