Multiple Hydraulic Fracture Stimulation in a Deep Horizontal Tight Gas Well
Authors: Abou-Sayed I.S. , Schueler S. , Ehrl E. , Hendricks W. 
Published: June 2014

This paper outlines the stimulation program used to successfully complete the 15,687 ft (4,783 m) TVD deep horizontal well Soehlingen Z-10 in the tight Rotliegendes sandstone onshore Germany. The program included the breakdown multiple rate injectivity test, use of proppant erosion stages to reduce near wellbore restrictions, and minifrac data analysis. Use of open annulus to analyze the stimulation treatment and modify the design is also covered. Considerations and experiments behind the design and execution of this horizontal well stimulation with hydraulic fractures perpendicular to the wellbore are presented and discussed. The well is now producing at a stable rate of 18 MMSCFD (20,000 m3/hr).

Introduction

Mobil Erdgas-Erdoel GmbH has recently drilled, completed and fracture stimulated a very deep horizontal gas well in an ultra-tight Rotliegendes Sand at a TVD of 15,687 ft (4,783 m) in North-West Germany (Z-10 well location is shown in Figure 1). Mobil is the operator of a consortium that includes BEBErdgas und Erdoel GmbH, RWE-DEA AG and Wintershall AG. The well was drilled in the Soehlingen gas field to a TVD of 18,860 ft (5,750 m) including a 2,063 ft(630 m) horizontal section. A multiple frac concept was used in order to develop an innovative method to economically produce significant gas resources from the tight sand.

The first experience with multiple fractures from an inclined well dates back to the 1960's when Mobil completed a field experiment. Fracturing horizontal oil wells has been extensively used by MAERSK in the Danish part of the North Sea. Also, NAM has stimulated a horizontal well with two fractures in a moderately permeable (about 1 md) gas reservoir.

Multiple fracturing of an ultra-tight gas sand (0.01-0.02 md) at a TVD around 15,700 ft (4,800 m) with a reservoir pressure of about 8,700 psi (600bar) and a temperature of 288 F (l42 C) has not been reported to date. This paper summarizes the fracturing concept and results of this unique project, concentrating on an integrated team approach with detailed frac design and project implementation.

Background

The multiple frac concept was used in order to economically produce significant gas resources in an ultra-tight Rotliegendes sand. Mobil, as an operator, began developing the Soehlingen field in 1982. The Main Sand, the target of this project, is a member of three productive sands in the Soehlingen field. The areal extent of the Main Sand is large (ca. 12 square miles, 30km2), The sand is massive (about 350 ft, 100 m) with moderate porosity (10 to12%) but with a very low permeability (0.001 to 0.1 md). The very low permeability is caused by the presence of diagenetic illite in the pore space, which reduces the permeability significantly. The Main Sand is underlain by a130 ft (40 m) fanglomerate layer separating it from the lowermost gas producing horizon. At the top of the reservoir a 30 ft (9 m) thick shale layer provides a seal to the Ebstorf Sandstone which is usually extremely tight.

This tight reservoir has been produced on a limited basis from three vertical wells drilled in the early 1980's and fractured using massive hydraulic fracture technique, i.e. with large fluid volumes (500,000 gallons of crosslinked fluid), large amounts of proppant (in excess of 1.2 million pounds), and up to 7 ppg proppant concentration. These wells typically produced about 0.09 MMSCFD (100 m3/h) after perforation and around 0.9 MMSCFD (1,000m3/h) after formation breakdown with acid. Post-frac rates were around 4.5MMSCFD (5,000 m3/h) sustained for up to six years but proven uneconomic. To economically develop this significant resource required production rates three to five times the rate of a conventionally fractured vertical well. To achieve this goal, a multi-fractured horizontal well was deemed necessary.