Exploratory well tests typically require timely, yet accurate log analysis from the responsible formation evaluation specialist. A rapid turnaround in the analysis is demanded due to economic considerations and decisions that are routinely made from log-derived, in-place reserve estimates. In general, in-place estimates are quickly expanded to include the reservoir's areal extent that was likely conceived from prior seismic and geological studies. Wireline data usually provide the porosity, saturation, pay thickness, and evidence of producibility that are input into reserve calculations. Core data are seldom available on exploration tests, especially in carbonate reservoirs.
A flexible algorithm defines formation resistivity factor (FR) boundaries within which log measurements from carbonates occur. Evidence shows the n exponent in carbonate reservoirs to be strongly affected by wettability characteristics and porosity type, but tolerable estimates of n can be made to obtain relatively accurate and timely estimates of saturation.
Most log analysis literature has been devoted to clastic reservoirs, although the majority of today's oil production comes from carbonate rocks. To date, carbonate reservoirs have provided nearly half of the produced hydrocarbons. Well-log measurements, at any depth level in a borehole, respond to rock properties, fluids or gas, and pore space.
Log responses are also a function of rock facies characteristics, and log portrayal of facies images is restricted by the limitation or improved by the completeness of necessary log measurements. Specific petrophysical parameters tend to occur in specific environments and particular facies variations. The most fundamental advantage of wireline logs is characterization of a continuous subsurface record of all stratigraphic units penetrated. Measured depth, thickness, and a qualitative comparison of permeable vs. impermeable beds are quickly identified. Caliper measurements allow for continuous monitoring of the borehole size, shape, and condition. In carbonates, calculated saturation values remain skeptical, but the major contributors to error are petrophysical exponents m and n.
The carbonate envelope provides freedom to quickly integrate geological information into log interpretation Experience shows the envelope to encompass the range of petrophysical parameters that are universally encountered in carbonate reservoir rocks, eliminating many laborious tasks, traps. and assumptions that often lie in wait. Typically, carbonate reservoirs are essentially shale-free; therefore, a rudimentary knowledge of the reservoir rock and porosity type is all that is needed to set up the algorithm. Well-site geologists can normally provide a general description of both rock type and porosity type. If an adequate cuttings description is not available however, today's rotary sidewall coring tools can obtain cores from selective depths. Instead of rock specimens, imaging devices often provide an adequate description of the porosity type.