When drilling in southern Alberta near the town of Strathmore, operators have historically encountered unpredictable lost circulation during cementing operations on many wells. With the surface casing set above ground water, operators are required to bring cement to surface on all production casings. Costly remedial cement treatments are often required. Openhole logs in the area have shown the presence of numerous coal seams at depths ranging from 20 to 250 m below surface. Post-cementing temperature logs indicate that the cement top may be found at any one of these coal seams. The presence of the coal seams alone does not, however, appear to definitively indicate the potential for a problem, as evidenced by wells separated by less than a mile that may react completely different. Currently, there is no way to determine which wells will experience problems so the normal approach has been to bring cement to surface before dropping the top wiper plug and displacing the casing. Unfortunately, this methodology has not proven to be 100% successful, and wells with no losses require the disposal of a significant amount of cement at a substantial cost.
Recently, the introduction of specifically sized silicic fibers to the cement blend in conjunction with a revamped cementing procedure has proven to be a cost-effective solution to the problem of low cement tops. This paper will take a look at previous efforts at solving the problem along with alternatives that have been considered and rejected. It also includes data showing the efficacy and economics of the new approach.
Shallow gas drilling in southeastern Alberta poses a unique set of challenges. Because the production rate on shallow gas wells is relatively low and the number of wells required to fully exploit the reserves is quite high, the driving forces behind drilling and completing the wells all revolve around reducing costs and improving efficiency. To put this into perspective consider the following: The time from spud to rig release is normally less than 24 hours per well. Typical production from a shallow gas well in southeastern Alberta ranges from 50 to 150 mcf/day. In 2001 there were more than 5,000 shallow gas wells drilled in the Western Canadian Sedimentary Basin.
One of the challenges that is often encountered in shallow gas drilling is lost circulation while cementing. Lost circulation while cementing creates additional costs in shallow gas drilling in several ways. Low cement tops require top-out cementing and in some cases very costly squeeze cementing operations. Cementing programs are often designed with large excess cement volumes in an attempt to achieve cement back to the surface despite losses. When excess cement is returned to surface, it must be disposed of at a substantial cost. Therefore the ability to predictably eliminate lost circulation while cementing will do away with the cost of top-ups and squeezes, reduce the amount of cement required and minimize surface disposal costs for cement returned to surface.