Reservoir characterization is important to EOR and numerical simulation studies, especially for complex naturally fractured reservoirs with low-permeability matrix rock. Solvent movement and volumetric sweep are controlled by fracture distribution and connectivity and by matrix permeability variation. All these factors were considered in our reservoir characterization and simulation study.The reservoir characterization was so effective that, in a 63-well simulation study, 70% of the wells were history matched on the first simulation runs without modification to the original geologic/petrophysical data. This can be attributed to the team approach of using geological, reservoir engineering, and petrophysical input.

The Weyburn Unit, located in the southeastern corner of Saskatchewan, has over 80 horizontal wells. These wells are producing from a complex, naturally fractured reservoir that has been on waterflood for over 30 years. Accurate prediction of horizontal well performance is required for waterflood development planning, facilities design and marketing. For more precise estimates, simulation modelling has been the tool of choice to incorporate the reservoir anisotropy and the effect of having a multi-layered reservoir. Specific area models were built, but the Weyburn Unit spans 83 Sections with over 160 potential horizontal well locations making full field simulation impractical. Therefore, analytical models, because of their flexibility, were used to estimate the performance of horizontal wells field-wide. To validate predictions from both the simulation and analytical models, all available information sources were combined. These included:off-trend vertical well performance,existing horizontal well performance in the Weyburn Unit and other Midale beds pools, andgeological models. As a result of this work, we were able to calibrate a published analytical model to better predict horizontal well performance in this complex reservoir and to optimize the design of future' wells. Introduction The Weyburn oil pool, located in southeastern Saskatchewan, was discovered in 1954 (Figure 1). By 1960, the field was fully delineated with vertical wells on 32 ha (80 acre) spacing. In 1964, it was placed on waterflood with 151 inverted nine-spot patterns. A year later, oil production peaked at 7,500 m3/d and then declined until 1985. Production decline was arrested with the drilling of 157 vertical infill wells from 1985 to 1992. About 27% of the 178 106m3 original-oil-in-place has been produced to date. A medium gravity crude oil (27 ° API) is produced from the Midale Beds of the Mississippian Charles Formation. These beds are divided into an upper chalky dolostone called the Marly and a lower limestone called the Vuggy. The original inverted nine-spot waterflood has been successful in sweeping the Vuggy, but has not been efficient in sweeping the Marly. The vertical infill wells targeted the Marly zone. By 1991, the number of potential economic vertical infilllocations was diminishing, so the Unit turned to horizontal drilling technology for further development. During 1991 and 1992, six horizontal wells were drilled, experimenting with different geological environments and well orientation. The lessons learned from this pilot were used in the design and execution of the commercial horizontal well development in 1993. Through a cost/benefit analysis it was decided to drill long horizontal wells (1,000 m) that parallelled the fracture trend. These wells were drilled underbalanced. This development drilling is expected to continue throughout most of 1995 bringing the total number of horizontal wells in the Weyburn Unit to over 80. Accurate forecasts of horizontal well production rate and reserves allow:a proper assessment of economic reward with infill drilling,a framework in which capital and operating costs can be controlled, andlead time to arrange oil transport to markets.

In excess of 100 horizontal wells have been drilled underbalanced in the Weyburn Unit in S.E. Saskatchewan. Initial underbalanced drilling (UBD) operations intermittently placed horizontal wellbores in an overbalanced condition due to large variations in bottomhole pressure while drilling. Pressure fluctuations acting at the sandface were found to have resulted in reduced productivity in many of these wells by forcing drilled fines into productive pore throats. In an effort to reduce formation damage, UBD procedures were modified to better maintain an underbalanced condition and decrease the magnitude of bottom-hole pressure surges occurring during drilling operations. Initial steps taken to optimize Weyburn UBD operations included the use of electromagnetic survey tools, the acquisition of bottomhole pressure data and adding on-site UBD technical support. This led to the implementation of basic operational practices such as minimizing survey and connection times, displacement of the drill string past the first float to nitrogen prior to making connections, shutting in the annulus during connections, and adjustment of circulation parameters to account for formation fluid influx, penetration rates, and wellbore length. Wells drilled using the adjusted UBD practices resulted in approximately 40% more production, on average, than those wells drilled using the initial UBD practices. Introduction During conventional drilling operations the hydrostatic pressure exerted by the column of drilling fluid is, by design, greater than the formation pressure of the zone being penetrated. This overbalanced pressure differential promotes both drilling mud filtrate and particle invasion into the target formation and can reduce well productivity. Fortunately, stimulation techniques can reach beyond damaged zones in most cased wellbore applications to provide access to undamaged reservoir. Horizontal well bores, however, are generally more susceptible to drilling induced damage than vertical wellbores due to their increased exposure time to drilling fluids and increased circulating pressure losses as horizontal length increases(1). Many horizontal well applications also involve underpressured or pressure depleted reservoirs, and are frequently completed open hole. Damage removal in these wells is often costly. Underbalanced drilling (UBD) has emerged as an effective technique to reduce weIIbore damage, particularly when drilling horizontally into underpressured formations. UBD refers to the drilling process involving downhole circulation systems designed with a hydrostatic pressure lower than the pressure of the reservoir section being penetrated. Properly designed, the process results in a continuous and controlled influx of formation fluids into the wellbore while drilling. Although both the magnitude of wellbore damage and the impact that damage has on well productivity are difficult to quantify in most horizontal wells, laboratory evaluations(2) have measured the effect that overbalanced, balanced, and underbalanced conditions can have on formation damage. These tests have demonstrated that underbalanced pressure conditions can significantly reduce the potential for formation damage both y mud solids blocking pore throats and by filtrate invasion. Field tests(3) have similarly identified benefits of UBD.

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. P. 707

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