Abstract

From 1991 to the end of July 1996, 105 horizontal wells with a total of 175 lateral legs have been drilled in the Weyburn Unit, Southeast Saskatchewan. They produce from a complex, naturally fractured carbonate reservoir that has been under waterflood for over 30 years. PanCanadian, as operator, has drilled the majority of these wells underbalanced using a nitrified water system. Although these wells were drilled in an underbalanced mode, reduced well productivity in some of the early phases of the infill program lead to the conclusion that formation damage may be occurring during drilling. In some cases horizontal well productivity was less than that of a vertical offset well. As a result, a three well pilot was implemented to evaluate the effectiveness and applicability of horizontal well stimulation technology in the Weyburn Unit. An average per well incremental oil rate of 27 m3/d provided the economic justification for a subsequent 20 well program. Candidate wells were selected based on a variety of geological and engineering criteria. Once selected, considerations such as pay quality, proximity to water zones, available injection support and lift equipment limitations were incorporated into the job design specifications for a particular well.

Introduction

The Weyburn field is located 130 km south-east of Regina, Saskatchewan, Canada (Fig. 1). It occupies approximately 180 km2 and contains over 178 E6 m3 of oil in place. Weyburn was discovered in 1954 and was fully delineated with 32 ha vertical wells by 1960. The field was unitized in 1962 and placed on waterflood in 1964 with 151 inverted nine-spot patterns. A year later oil production peaked at 7500 m3/d and subsequently declined until 1985. Production decline was then arrested by a 157 well vertical infill program implemented from 1985 to 1992. By 1991 the number of economic vertical wells was decreasing and the Unit turned to horizontal drilling technology.

A six well horizontal pilot program was conducted in 1991 - 1992. The success of the pilot lead to the commercial scale horizontal well drilling program which began in 1993 and is active to date. The advancements in drilling technology and the understanding of the reservoir have allowed for a progressive reduction in reservoir access costs from 1991 to present. This has enabled the Unit to economically develop the lower productivity areas of the pool, which in 1991 were thought to be non-prospective. In the initial stages of the horizontal program short (350 m) single lateral, single build legs were drilled. This type of drilling continued for 66 additional wells, however, horizontal pay averaged in excess of 1000 m. A typical horizontal well would have 177.8 mm intermediate casing set in the vertical section at around 1375 m. A medium radius build section (250/30 m) was utilized and a 158.8 mm open hole section was drilled approximately 1000 m horizontally. Average formation depth is 1450 m. After completion of the single lateral program the Unit turned to single build, dual lateral wells. The first leg was drilled approximately 1000 m horizontally. The second leg was kicked off in the horizontal section of the first leg and drilled parallel to and of approximate equal length to the first with a separation of 150 m. Wells currently being drilled involve two build sections from a single vertical wellbore each 1800 apart. Each build section would have a dual lateral drilled as described above. This type of well is called a quad leg horizontal well and penetrates approximately 4000 m of pay. There are presently 105 horizontal wells with 175 lateral legs. Horizontal wells consist of single, dual, tri and quad leg wells. Current daily oil production is in excess of 3600 m3/d, approximately 60% of which is from the horizontal wells.

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