Introduction:
Reservoir fluid pressure testing and sampling are critical components of wireline operations that significantly impact the quality and usefulness of the data obtained. The selection of pressure test and sampling points is a key decision that operations geologists must make. This article will discuss the factors that operations geologists consider when deciding on pressure testing and sampling points.
Factors affecting the selection of pressure testing and sampling points:
- Well Type: The type of well is a major factor that operations geologists consider when deciding on pressure testing and sampling points. Different types of wells have different objectives, and this is reflected in the number and location of pressure tests and samples taken.
• Wildcat and exploration wells are drilled in an area where there has been very little or no hydrocarbon exploration. The primary objectives in these wells are to determine the presence of hydrocarbons, to establish the extent of reservoirs and to observe characteristics of reservoirs and the fluid contained in them. Taking crucial decision whether or not to continue further exploration requires large amount of wireline log data including pressure tests and fluid sample data. Therefore, a large number of pressure tests and fluid samples are taken in wildcat and exploration wells.
• Development and Production Well: After a successful campaign of wildcat and exploratory drilling, development and productions wells are drilled. Here the main objectives are to optimise and maintain production and to identify any problem with production. By the time development wells are drilled, we have significant amount of data on reservoir extent, reservoir characteristics and fluid characteristics. Therefore wireline testing and sampling becomes limited and focussed to specific zone of interest and for specific purposes such as production optimisation and understanding reservoir compartmentalisation etc. In other words fewer tests and samples are taken in development and production wells compare to wildcat and exploration wells. - Geological Considerations:
• Formations and thicknesses: Pressure tests and sampling are conducted only in reservoir rocks; all other rock types are ignored. Testing thin reservoirs can be challenging, especially if porosity and permeability are poor. In such situations, accurate depth correlation is performed frequently. A single test is sufficient in thin reservoirs (less than 5 ft thick). Multiple tests are often conducted in thick reservoirs.
• Density and porosity: Low-density and high-porosity areas offer more chances of success than tight spots in the reservoir rock. Neutron, density, and sonic logs help avoid tight spots. (Tight spots are areas in reservoir with high shale content or high density)
• Proving Possible and Probable reservoirs: Based on quick-look method, hydrocarbons are divided as proven, possible, and probable. One of the objectives is to convert probable and possible net pay into proven net pay. This is done by establishing cut off on resistivity, plotting pressure gradients and by taking fluid samples. Accordingly pressure test points and or samples are selected.
• Identification of fluid and contacts: Multiple fluid types may exist in thick reservoirs; therefore, multiple test points are selected to establish pressure gradients to identify fluids (gas-oil-water) and their contacts. A shift in pressure gradient trend indicates reservoir compartmentalisation. - Wellbore Challenges:
• Potential problem zones: Such as caved and washed out zones that frequently result in seal failure are usually avoided by operations geologists. Caliper log help in identifying such spots. Similarly high dogleg and extreme deviation sections are usually avoided unless these are critically important, in which case, care is taken to avoid tool or cable getting stuck.
• Tight and sticky hole: This Condition apart from being prone to sticking tool and cable also causes loss of depth correlation and lead to incorrect probe placement, resulting in tight tests. Again such locations are avoided for obvious reasons. However, if critically important test points and or sampling point fall in this zone; Then one should ensure good depth correlation to avoid tight tests and take precautions to avoid tool and cable sticking. - Conclusion: Selection of pressure test points and sample points is heavily depend upon the type of well, being drilled. Reservoir rocks in wildcat and exploration wells are extensively tested not just to prove presence of hydrocarbon but to collect large amount of reservoir and fluid data for future use.
In view of high cost of operation, comparatively less number of pressure tests and sample points are selected to address specific needs in development and production wells.
In all type of wells, points are selected to remove ambiguity in log interpretation as well as to calibrate water saturation calculations. Another significant objective is to remove or at least reduce number of possible or probable hydrocarbon net pay by clearly identifying the fluid. Also during the operation attempt is made to establish resistivity cut off so that above certain resistivity threshold a reservoir can logically be proven to be hydrocarbon bearing for certification purpose. Similarly a density or porosity cut off is established as a rough guide for future use to see at what density level the reservoir test will fail or be successful. So that tight looking points can be avoided to optimise time and cost.
Sometime even important looking points for pressure testing and sampling are avoided if they fall in washed or caved section of hole or if a serious dog leg exists or if the hole is showing sticky tendencies to avoid test failure and complications.
In each operation reservoirs are usually tested in priority order from most promising to least promising. So that we have most valuable data in our possession in case operation has to be abandoned due to deteriorating hole condition or tool malfunctioning.
Operations geologists and wellsite geologists consider these and a few other local factors while selecting and conducting pressure tests and samples. This strategy helps minimise risks, collect good data and gain understanding on reservoir characters at minimum cost.
