Multi-Layer Test Interpretation

In order to use pressure transient data to evaluate each layer in a commingled system, some form of selective testing is required. A single test of the composite behavior of all the layers not only fails to give information about each layer's properties, but it can also give erroneous information about the total system properties, typically underestimating both the skin factor and the permeability.

As an alternative to isolating each zone and testing individually (e.g. straddle test), the multi-layer test (MLT) has been developed whereby the flow rate from each layer is measured using a spinner flow meter during a pressure transient test. Variable rate superposition analysis can then be applied to the data from each layer in turn to identify the appropriate reservoir model and to obtain an estimate of K, S, Pe, and other model parameters.

MLT principles can be described using the schematic shown in Figure 1 shown below. Usually four basic parameters must be identified for a commingled reservoir system; Kh_i, S_i, Pe_i and D_i (for gas reservoirs or high flow rate oil producing oil wells). Attempts to identify these parameters have historically been performed by three methods: 1) isolation testing, 2) conventional variable rate testing, and 3) production logging (PL).

The alternative MLT method derived from PL tests involves the sequential measurement of flow rate and pressure transients from an individual layer or group of layers after a rate change, starting preferably with the bottom layer and working up layer by layer; e.g. 1) bottom-most layer only – rate change, 2) bottom-most layer plus second layer - rate change, 3) bottom-most, second, and third layers – rate change, etc. This data acquisition process has been referred to as transient multi-layer test or simply MLT.

Figure 1: Schematic of a Layered Reservoir System

MLTs have the advantage that they can be carried out on a production well without pulling the completion and a production logging job is much more cost effective. Another advantage is that the total production from all layers is measured, whereas a series of straddle tests may in fact be testing the same zone, if the layers are not hydraulically isolated.

MLT data are crucial for reservoir management in commingled systems where more than one layer is producing and cross-flow may be occurring between them. Good reservoir understanding is crucial, particularly for reservoir simulation, voidage control, pressure maintenance, and workover decisions.

eP can provide MLT design and interpretation services using in-house software. Interpretations are typically processed in less than two weeks, depending on data quality, geological and reservoir data availability, and good coordination and communication with the operator and/or service companies, especially in remote locations.

eP has industry recognized experts in PLT/MLT analysis who have worked for many years with prominent wireline and logging companies, and have authored a number of relevant papers. eP also provides external training and in-house courses in subjects related to multi-layer tests such as production logging, well test analysis, etc. to service company and operator personnel worldwide.

MLT Interpretation Data Requirements

eP recognizes that the following data requirements are not fulfilled in many cases. However, the data ideally required to provide the most complete analysis include:

• Interpreted production logs
• Interpreted open hole logs
• Well schematics for test operations including equipment locations
• Well deviation data over test interval range
• Detailed sequence of events for tests
• Surface test reports from test contractor
• Surface set up for test
• PVT data of produced fluids
• Pressure gauge files on correct time baseline in electronic format
• Production log data files on correct time baseline in electronic format
• Top structure map
• Reservoir layering scheme
• Core data (specifically including total and effective permeability)

To enable rapid project turn around, these data are best supplied in electronic format and sent by Internet e-mail, if possible.

MLT Interpretation Methodology

eP will perform analysis of the multi-layer, multi rate test to obtain layer parameters by standard techniques using rate de-convolution.

• Review and confirm PLT analysis. eP would not perform the PLT interpretation at this stage unless requested.
• Calibrate PLT spinner response
• Perform selective inflow performance analysis (SIP) - see Figure 2 below

Figure 2: SIP Analysis

• Correct individual station pressure data to a common datum depth
• Splice flow periods into continuous record - see Figure 3 below

Figure 3: Raw Data of a MLT Test

• Perform MLT analysis using variable rate superposition and rate de-convolution to extract layer parameters
• Confirm individual layer analyses by using advance simulation within PanSystem - see Figure 4 below
• Report test, results, and conclusions

Figure 4: Confirmed Pressure Match for a Two-Layer Model by Using Advance Simulation in PanSystem

Reporting MLT Interpretation

A comprehensive MLT 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 pressure and rate matches obtained from the pressure analyses performed in PanSystem, as well as the calculated reservoir parameters for each layer and reservoir model features.

Recent MLT Interpretation Projects

• Multi-layer well test analysis for an operator in North Africa, where an oil well with four perforated zones was tested for 76 hours. Starting with a build-up, drawdown, and a second build-up, a multi rate test was performed at different choke settings. Reservoir parameters such as permeability and skin factors were calculated for each of the four layers in the system.
• A number of MLT analyses were recently carried out for an operator in an offshore field in West Africa, where most of the wells presented complex multi layer systems. Cross flows detected by PLT tests were successfully simulated during MLT analyses in order to obtain meaningful reservoir properties.