convection requires a sufficient thermal gradi- ent, or inverted density gradient, to effect change in the fluid distribution. These normal processes commonly result in gravitationally ordered fluid gradients progress- ing upward from heavy oils, medium oils, light oils, condensate, wet gas, lighter gas and finally to methane. However, nonequilibrium conditions often exist—even given geologic timescales for fluids to equilibrate. Fluid mixing processes in the reservoir may be extremely slow. The added effects of tectonism, faulting and reservoir het- erogeneity contribute to complicated fluid distri- butions. The processes acting on reservoir fluids can preserve a nonequilibrium condition.


Another contributor to nonequilibrium condi-


tions is biodegradation, which occurs at the oil/ water contact (OWC). Biodegradation results from the metabolic conversion of saturated hydrocarbons, primarily by methanogenic and sulfate-reducing bacteria in anoxic conditions. Preferential removal of alkanes at the OWC by biodegradation yields an increased asphaltene concentration causing large, nonequilibrium viscosity gradients. The OWC may change with subsequent charging of the reservoir or with seal leaks, but biodegradation remains active only below about 80°C [175°F]; above this tempera- ture the microbes are no longer viable. Among other effects, biodegradation raises oil viscosity, lowers API gravity, increases asphaltene and sul- fur content, and increases concentrations of met- als.12


Biodegradation can exert major control over Power cartridge


the quality of the oil as well as its producibility.13 Biodegraded oil may be found as a mix of oils.


Sample-bottle module


For example, primary oil arrives first, is biodegraded, and is followed by oil from subsequent reservoir charges. The secondary oils may be unaffected, appearing after biodegrada- tion has ceased, creating spatial variations in fluid properties. In addition, biogenic or thermogenic gas


Sample-pumpout module


InSitu Fluid


Analyzer module Hydraulic module


may override existing oil in the reservoir, move updip and disrupt the existing reservoir fluid gradients.14


with this influx, creating compositional varia- tions.15


Detection of these gradient disruptions


from charging and recharging may indicate the presence of compartments, a topic to be dis- cussed later. Ultimately, rather than an open container


Quicksilver Probe tool


Guard-fluid analyzer module


Guard-pumpout module


filled with layers of water, oil and gas, the reser- voir is a complex architectural structure contain- ing mixtures of fluids. There is no single tool to identify these complexities, and engineers create completion strategies and reservoir development plans using data from many sources. DFA mea- surements, however, have proved highly effective as a tool for understanding both reservoir fluids and architectural complexity.


> The MDT tool. The InSitu Family service is delivered downhole by the MDT tool. Along with the InSitu Fluid Analyzer module are the Quicksilver Probe tool for quick fluid-sample cleanup, dual pumpout modules for flowing sample- and guard-probe fluids and a sample- bottle module. Recovered samples are used for surface-laboratory analysis of reservoir fluids.


Application-Driven Innovation Wireline formation testing tools (WFTs) first appeared in the 1950s as a means to retrieve fluid samples for surface analysis. Laboratory testing of these samples was hampered by contamina- tion, particularly with filtrate from the drilling fluid, and by alteration of the fluids during the sampling and transfer process. Successive tool generations led to the development of more- advanced tools, such as the MDT modular formation dynamics tester, which incorporated


42 The GOR of the primary oil changes


innovations such as multiple chambers, the abil- ity to pump fluid into the wellbore before captur- ing a sample, improved accuracy and resolution, a variety of probe styles, dual-packer assemblies and focused sampling to significantly reduce mud-filtrate contamination (below left). The MDT tool is also the primary platform for fluid property measurements. Reservoir engineers need accurate assess-


ment of fluid properties for reservoir evaluation, flow assurance, reservoir simulation and model- ing, facilities design, production strategies, reserves calculations and recovery estimates. Early sampling methods sometimes yielded sub- optimal results. Relatively few samples were used with simplistic fluid models to explain fluid dis- tributions in the reservoir. In addition, engineers resorted to analytical methods to correct labora- tory measurements for phase changes and mud- filtrate contamination, which often led to erroneous fluid characterization. This limitation has been partially overcome by the ability to pump contaminated fluids from the formation prior to sample initiation. The MDT tool’s pumpout module is used to


flow reservoir fluids into and through the tool. This enables reduction of filtrate contamination to obtain nearly virgin native fluids, as deter- mined through the DFA measurements, as well as the acquisition of reservoir fluids in sample bot- tles carried in the tool. One such operation in Kuwait pumped 2,100 liters [555 galUS] over a 66.5-hour interval to acquire uncontaminated samples. Although the volume of moved fluid is considerable, this is not an efficient method if multiple samples are needed or if DFA fluid profiling with multiple test points is the goal. A focused-sampling probe, added to the MDT


tool in 2006, greatly improved wellsite efficiency, allowing the timely acquisition of fluid samples free or nearly free of mud-filtrate contamina- tion.16


Using a concentric sampling arrangement


and two synchronized pumps, the Quicksilver Probe tool acquires uncontaminated samples in a much shorter time frame (next page, top). An outer guard ring extracts fluids—primarily fil- trate and contaminated formation fluids—that enter the probe peripherally. Fluid flowing through the central probe quickly transitions from filtrate-contaminated fluids to formation fluids of acceptable quality for in situ fluid prop- erty measurements. Low-contamination fluids are quickly avail-


able for downhole analysis and more samples can be taken in a reasonable time frame. Tool sensors and fluid analysis capabilities have also advanced


Oilfield Review


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