Stimulation of Horizontal Wells In Shell Canada Limited
Authors: Jones P.W. , Konopczynski M.R. , Milligan M.R. 
Published: March 2010

The design and results of stimulation treatments in Shell Canada's horizontal wells to 1993 are discussed. Shell Canada's experience with horizontal wells started in 1988 with three wells drilled in the Midale Field, Saskatchewan. Application of horizontal technology grew and over 20 new horizontal wells or reentries were drilled in Shell's oil and gas fields during 1993. Horizontal drilling continued to be a major part of the subsurface capital program in subsequent years. The stimulation techniques used on the carbonate reservoirs of those horizontals varied dependent on well type, field characteristics and economics. The first horizontal well at Midale was cased, selectively perforated, and acidized. Most subsequent horizontal wells were, however, completed openhole. Shortcomings in the stimulation technology and efforts undertaken to improve stimulation results are discussed.

Introduction

Effective stimulation of horizontal wells presented some new challenges. The main challenges were:Identification of the hydrocarbon bearing and potentially productive intervals along the horizontal wellbore;Effective placement and diversion of acid to ensure the treatment of all productive intervals along the horizontal wellbore;Identification of the nature and magnitude of the damage to be removed.

Shell Canada made extensive use of coiled tubing to stimulate its horizontal wells. This was due to coiled tubing's (CT) proven ability to be run to the end of the horizontal section and withdrawn while pumping/jetting acid and other treatment fluids across the zones of interest. Economides(1) had provided guidelines on acid treatment volumes per unit length of horizontal section, pump rates and withdrawal rates for CT to ensure effective treatment across the horizontal sections.

Because of the difficulties experienced in removing damage from some horizontal wells using acids and the increased water production that was sometimes experienced, underbalanced drilling was identified as a means to minimize drilling damage(2). Underbalanced drilling was subsequently employed in Midale and Harmattan East. Whilst underbalanced drilling did provide benefits in certain settings, the incremental cost to drill underbalanced vs. effective stimulation later to remove damage was an important consideration.

Stimulation of Midale Horizontal Wells

Horizontal wells in the Midale oil field, located in SE Saskatchewan, presented unique challenges in stimulation design. This was due to the binary nature of the reservoir geology and to the maturity of the waterflood. The Midale formation from which hydrocarbons are produced is composed of a dolostone, commonly called the Marly, overlying a limestone, called the Vuggy. Imprinted on this geology is a fracture network trending southwest- northeast. Because of the higher porosity and more plastic nature of the dolostone, the Marly tends to be less fractured than the underlying Vuggy. While it might be anticipated that this fracture network would provide extensive communication between Marly and Vuggy, thin and laterally extensive barriers of tight limestone exist through out the field.

The foregoing geologic factors greatly influenced the waterflood response of the Midale Unit. The field was developed on an 80-acre inverted 9-spot pattern, with a variety of infill wells for accelerated production.