The Permian Basin is a mature oil province where many fields are under secondary and tertiary recovery. Waterfloods abound and it is not uncommon to have wells that make 20 BOPD and 2000 BWPD. At today's oil price the economics of these fields are marginal on a good day. The operators of these reservoirs would like to be able to shut off some of the water production without losing a drop of their meager oil production. Ideally, the drop in water production would affect relative permeability and result in an increase in oil. The only technique available to shut off water has been the "plug-and-see" (also known as the hunt-and-peck) method. Experience with this technique indicates that in 60% of the cases either the water cut increases or the oil production decreases.
If the reservoir engineers better understood where the oil and water were coming from they would be more successful in using plugs to decrease the water flow. There have been two major roadblocks to the successful application of production logging. First, the wells do not flow naturally so pumps are used. As a result the production logging string cannot get to bottom. Second, even if tools could be placed below the pump, conventional production logging tool strings cannot accurately detect oil cuts below 5%. Until the introduction of the new PS Platform* production services tool string, West Texas waterflood operators had no way to accurately determine what was happening downhole.
The sensors on the PS Platform string not only measure very low oil cuts, but the tool string has also been placed below electronic submersible pumps (ESPs) in an ingenious rig-up technique that required a joint effort between Schlumberger Wireline and REDA. Once the pump is turned on and flow stabilizes, the sensors can measure very low oil rates accompanied by high water rates.
This paper describes the use of the new technique on two wells in the Altura Midland Farms Unit. In both cases the PS Platform was deployed below an ESP and delivered high-quality answers that significantly impacted the economics of the field. These successes consequently lead to the use of the PS Platform sensors, in conjunction with TDT* Thermal Decay Time tool, in tertiary CO2 floods.
In 1989 Altura conducted a plugback program on 25 producing wells in their Midland Farms Unit. Before the program, these wells produced 847 BOPD and 25,515 BWPD; an average of 34 BOPD and 1021 BWPD per well. The plugbacks cost Altura $208,000 ($8000 per well). After the program, these wells produced 914 BOPD and 15,560 BWPD. The payout on the program was 6.1 months. Although this was an acceptable result, it obviously could have been better. Of the 25 workovers, 9 were successful and paid out within 6 months; 9 were actually better wells before the plug backs; 2 had payouts in excess of 5 years. This experience left Altura with a desire for a better technique to minimize water production in this field.