Prediction of large-scale communication in the Smørbukk fields from strontium fingerprinting

Abstract

This paper discusses a new method which uses intra-field variability of formation water chemistry as a measure of degree of reservoir compartmentalization. If two reservoir units are in good flow communication, it is more likely that any water compositional variations that might have arisen would have homogenized by mixing processes (diffusion, dispersion, flow). On the other hand, a lack of flow communication is likely to inhibit water mixing and thus preserve variations in water compositions. Thus, whatever the cause of intra-field variations in water composition, two reservoir units that have a similar water chemistry are more likely to be in good flow communication than two units with different water compositions. In this study the isotopic composition of strontium ( 87 Sr/ 86 Sr) dissolved in the formation water was used to monitor formation water compositional variability. This natural isotopic tracer is particularly useful as it can be measured simply from conventional core samples by extracting residual salts which have precipitated in the pore spaces as a result of formation water evaporation during storage. The Smørbukk fields (Smørbukk North and Smørbukk South, Haltenbanken area, Norwegian Continental Shelf) comprise three Jurassic reservoir intervals, the Garn, Ile and Tilje formations. In Smørbukk North the main problems are vertical compartmentalization by areally extensive shales and lateral compartmentalization by extensive quartz cementation. In one well, residual salt analysis (RSA) shows that water 87 Sr/ 86 Sr compositions are extremely homogeneous in each of the Tilje 1 and 2 reservoir zones, indicating good internal vertical communication. However, the Tilje 1 and 2 waters are significantly different from each other, indicating that the shale that separates Tilje 1 and 2 is an effective barrier to vertical communication. A similar relation is seen in a second well, suggesting that the shale barrier is indeed laterally extensive. In the second well, Tilje 1 RSA data are the same as the results for that zone in the first well, indicating excellent lateral communication between the two wells. In Tilje 2, however, there is a distinct between-well difference in formation water composition, indicating that lateral communication is poor in this zone. In Smørbukk South, lateral connectivity is further complicated by faulting. Using RSA data from three wells it was possible to predict that: the Garn has good internal vertical communication, but is laterally divided into two isolated areas, probably the result of a sealing fault; the Ile forms three laterally isolated compartments; the Tilje is subdivided vertically into three flow units that are interpreted to have poor communication with each other in two of the wells. Laterally, Tilje 1 and 2 form two flow compartments, probably due to faulting.