Unwanted water production in oil and gas wells is a limiting factor in the productive life of the well. Many factors are influencing the drive for improved water control loss of hydrocarbon production, environmental impact of disposal, government regulations, and public opinion. The environmental issues and costs related to produced water and it's disposal are becoming major burdens for producers. The economic factor of reducing water production far outweighs the cost of typical water control treatments. Historically, water control treatments have often failed due to one or more of the following problems:
the source of the problem was not properly identified,
the wrong treatment was carried out, or
the correct treatment was improperly performed.
Comprehensively identifying the source of the water control problem is the first and most important step in suggesting a solution. Well logs can be very effective at diagnosing downhole situations that can lead to unwanted water production. Casing leaks, cement channels, coning, and watered-out reservoirs are all typical sources of unwanted water that can be easily defined with well logs. Evaluating the effectiveness of water control treatments with cased-hole logs can help determine the success of the procedures. In many cases, ineffective treatments may need only a simple remedial second pass to accomplish the original goals. Specific applications are provided showing the use of open-hole, production logging, pulsed neutron, and casing/cement evaluation logs that detect and define downhole problems that can lead to the unwanted production of water.
Open-hole logs can be very useful in determining possible causes and sources of unwanted water or gas production. Caliper logs are good for finding areas where severe borehole washout has occurred that can contribute to poor cement bonding. Gamma ray and SP logs can be used to delineate shale beds from possible water or hydrocarbon producing reservoirs. Resistivity and porosity logs (sonic, density, neutron) can be combined to determine the location of water and pay zones which later can be compared to cased-hole logs to monitor changing water levels or look for coning in producing reservoirs.
Full wave sonic measurements can be combined with bulk density log data to predict fracture height as a function of the differential pressure between downhole treatment pressure and fracture closure pressure. The log shown in figure 1 shows a FracPressure analysis. Fracture height predictions were made prior to the treatment to ensure that job design limited fracture growth, avoiding the water table. Track 1 contains the gamma ray and borehole profile. Fracture pressure is the amount of pressure equal to the least principle horizontal stress, and is computed from the rock properties measured by sonic and density logs. This stress profile identifies barriers to fracture growth and stress contrasts between producing zones. Track 2 shows the calculated static fracture initiation and closure pressures. The fracture initiation pressure is the pressure applied to begin the fracture. Track 3 shows formation lithology as determined from a computing center open-hole log analysis.