Pilot Application of a Blocking Agent - Weyburn Unit, Saskatchewan
Authors: Mazzocchi E.F. , Carter K.M. 
Published: December 2007

Channelblock is a gelatinous blocking agent that permanently reduces channeling in waterflooding operations. It diverts injection water into the tighter zones and thus effects higher waterflood recovery. The pilot operation described here illustrates a successful application on the edge of a fractured limestone reservoir.

Introduction
The Weyburn Unit is situated in southeastern Saskatchewan and encompasses an area of approximately 100 sq miles. The field was discovered in Jan. 1955 and was extensively developed by drilling 675 wells on 80-acre spacing (Fig. 1). A distinctive feature of this Mississippian Midale Beds pool and of other fields in the same part of the province is the presence of an anisotropic permeability system whose major axis is oriented northeast-southwest (Fig. 2). This fracture system, which was first recognized on the basis of oriented fracture cores, has been clearly identified by waterflood performance. Oil accumulation in this stratigraphic trap is found mainly in the Midale beds of the Charles formation of Mississippian age. The oil reservoir is subdivided into two distinct lithological units; the upper is the Marly limestone, and the lower is the Vuggy limestone (Fig. 3). The Marly rock is a microgranular carbonate characterized by fairly uniform porosity averaging 26 percent and low permeability averaging 4 md. The percent and low permeability averaging 4 md. The Vuggy is generally microcrystalline, fragmental limestone with extreme variations in pore size and in porosity-permeability distribution. Porosity averages porosity-permeability distribution. Porosity averages 11 percent and the permeability may vary between 0.1 and 4,000 md. Oil gravity varies with structural position from a low of 26 degrees API at the southern position from a low of 26 degrees API at the southern downdip limit to a high of 35.6 degrees API at the highest structural position to the north. An inverted nine-spot injection pattern was selected for waterflooding this type of limestone reservoir. Such a pattern was chosen because it could easily be converted to a line drive or a staggered line drive either by suspending the uneconomical on-trend wells or by converting them to water injection service.

Flood Performance
The waterflood scheme was implemented in June 1964. The angle that the fracture plane forms with the line connecting adjacent producers and injectors is the most important factor affecting the flood performance of each producer. performance of each producer. Fig. 4 shows that the performance of an on-trend well is characterized by a premature water breakthrough, rapid oil decline, and a sharp increase in water production. For this category of wells the estimated production. For this category of wells the estimated total waterflood recovery is approximately 8 to 12 percent of the original oil in place. percent of the original oil in place. Fig. 5 illustrates the performance of an off-trend well belonging to the same injection pattern. There was a gradual response to the flood for 2 or 3 years, and then there appears to have been a long subordinate phase. phase. The Marly zone is generally not fractured. In this zone the values of the factors governing the flood are as follows: mobility ratio, 0.32; Lorenz coefficient, 0.45; and permeability variance, 0.55. The Vuggy pay is fractured and has an average mobility ratio, Lorenz coefficient, and permeability variance of 1.75, 0.78, and 0.87, respectively. Furthermore, the water and residual oil saturations in the Vuggy are higher than in the Marly, resulting in a lower flushing efficiency.
JPT
P. 973