Channelblock Pilot-Area Application, Weyburn Unit, Saskatchewan

Abstract

Abstract The results of treating a pilot area located on the east edge of the Weyburn Unit with Channelblock* are discussed in this paper. Channelblock is obtained by mixing polymer in low concentrations 0.05 to 0.1%) withwater and adding an "activator", which greatly changes the characteristics of the injected water to a gel-like material This gelled polymer gradually penetrates and adsorbs itself on the walls of the highly permeable streaks or fractures, thus diverting large quantities of water into tighter zones and resulting in a higher waterflood recovery. Introduction THE WEYBURN UNIT is situated in southeastern Saskatchewan and encompasses an area of approximately 100 square miles. The field was discovered in January, 1955 and was extensively developed during subsequent years by drilling 675 wells on 80-acre spacing (Fig. 1). A distinctive feature of this Mississippian Midale Beds pool and of other fields located in the same part of the Province is the presence of an anisotropic permeability system) with the major axis of permeability oriented in a northeast direction (Fig-2). This fracture system, which was first recognized an 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. It is subdivided into two distinct lithological units; the lower is called the "Vuggy" limestone and the upper is called the "Marly" limestone (Fig 3). The Marly rock is a micro granular carbonate characterized by fairly uniform porosity and low permeability. The Vuggy rock is generally a microcrystalline, frag-mental limestone with extreme variations in pore size and in porosity- permeability distribution. Oil gravity varies with structural position from a low of 26 °API at the southern downdip limit to a high of 35.6 °API at the highest structural position to the north~ An aquifer of modest dimensions is generally present along the edges of the field. An inverted nine-spot injection pattern was selected for water-flooding this type of limestone reservoir because of the flexibility of this pattern, allowing for the conversion to a staggered line drive or a line drive when the on-trend wells along the fracture plane water out and the flood front moves toward the off-trend wells. Since the implementation of the waterflood injection scheme in June, 1964, the on-trend producers ha.ve generally shown early breakthrough ollowed by a rapid productivity decline, a sharp increase in water production and what appears to be a very short waterflood productive life. The flood performance of the off-trend and corner wells is essentially affected by their degree of stratification. This unfavourable waterflood performance has been substantially improved in a pilot "rea of the field by the use of Channelblock chemicals. It is the object of this paper to give a description of the treatment and a detailed presentation of the results obtained. GEOLOGY The Weyburn field is situated along the northeastern flank of the Wilnston Basin in southeastern Saskatchewan.