PLT Interpretation

PLTs are carried out routinely on producing hydrocarbon wells to determine the source of oil, gas and water production, where the well has perforations in more than one layer, or over a large interval. Typically, the PLT tool string will be composed of one or more spinner flow meters, a pressure gauge, a temperature gauge, and a fluid density or capacitance tool.

PLT data are crucial for reservoir management in areas such as reservoir simulation, voidage control, pressure maintenance, and workover decisions.

PLTs enable the identification of fluid entry points and the type of fluids entering the wellbore, allowing decisions to be made relating to zonal shut-offs, additional perforations, re-perforating, etc.

PLTs may also be carried out on multi-layer injection wells to determine the proportion of injected fluid entering each layer or set of perforations.

Frequently, the interpretation of the acquired raw data will be carried out by the same service company which performed the well site wireline work. However, there are instances where, due to time constraints or workload, the logging company can not provide an accurate interpretation of the data for the field operator within an acceptable timescale. In some instances, the full value of PLTs has not been realized because oil company personnel often do not have the time to effectively specify, audit, and interpret increasingly sophisticated PLT programs.

Weatherford can provide PLT design and interpretation services using in-house software. Interpretations are typically processed in less than two weeks, compared to several months in certain instances from logging companies.

Weatherford has industry recognized experts in PLT analysis who have worked for many years with prominent wireline logging companies, and have authored a number of relevant papers. In-house expertise in the associated areas of cased-hole saturation analysis (e.g. TDT) allows a deeper understanding of reservoir and well performance. Weatherford also provide external training and in-house courses in production logging theory and practice to service company and operator personnel.

Weatherford has specific expertise in PLT analysis in horizontal wells. PLT data from such wells has been presumed unusable in many cases. Weatherford has determined the effects that different completions, geology, deviations, sensor types, and production regimes have on a number of conventional production-log sensors. This means that seemingly poor data can still provide viable interpretations.

PLT Interpretation Methodology

Ideally, the acquired raw digital data should be received by Weatherford in LAS format, although many other formats are acceptable. The data are then quality checked and loaded into Weatherford's PLOG in-house interpretation software.

Well data such as deviation angle, completion equipment, and perforated intervals, along with PVT data for the produced fluids, are also loaded into the program.

Calibration of the spinners is carried out by cross-plotting logging speed versus spinner rps, allowing fluid entry points and flow profiles to be identified.

Figure 1: Spinner Plot - 3 Up and 3 Down Passes at Different Logging Speeds with Well Flowing

Figure 2: Spinner Calibration Cross Plot

The density or capacitance data is then used to identify the fluid types and water holdup in the well.

Figure 3: Density Log - Up and Down Passes with Well Flowing

The temperature log is also examined to verify the interpretation and to check for additional anomalies.

Although the temperature data are not used in the flow profile calculations, examination of the temperature differences in the wellbore can confirm fluid entry points, and under shut-in conditions, can aid in identifying cross flow from one layer to another.

Figure 4: Temperature Log - Up and Down Passes with Well Flowing

If required, and if data is available at two or more flow rates, estimates of individual layer PIs and layer pressures can also be generated.

By plotting the flowing pressures versus the calculated fluid flows from each layer, the intercept on the pressure axis can be determined to give an estimate of the shut-in pressure in each layer, and the slope of the generated line provides an estimate of the layer PI.

Figure 5: Pressure Log - Up and Down Passes with Well Flowing

Reporting PLT Interpretation

A comprehensive report is provided to the client detailing the interpretation, with comments on the raw data quality, conclusions, and any recommendations drawn from the interpretation.

The report includes plots of the acquired logged data types, as in the figures above, and a plot of the final interpreted flow profile as shown below.

Figure 6: Total Flow Profile with Water in Blue & Oil in Green (2 Phase Production Well)

Recent PLT Interpretation Projects

• Review of PLT data acquired in highly deviated and horizontal wells from BP's fields in the North Sea and Alaska. Response patterns for measurement sensors in horizontal wells were identified to aid interpretation. Deconvolution techniques were used to interpret complex multi-phase flow patterns, and opportunities to improve existing measurement techniques were identified. The results of this study mean that data that were previously considered unusable can now be analyzed and provide new insights into horizontal well performance.
• Integrated PLT, TDT, and core analysis study to determine aquifer influx behavior in a Southern North Sea gas field.
• Several PLT analyses to allocate rates to layers for multi-layer test (MLT) interpretations in Europe and Africa.
• Numerous PLT analyses for many oil and gas companies throughout the world.