The Value of Wireline Formation Testing for Reservoir Evaluation

Introduction

Wireline formation testing is a well-known technology and have been widely used in the oil & gas industry for years to make properly a reservoir evaluation together with others tool including logs, cores, logging while drilling (LWD), drill sterm tests (DST), etc. Geoscientist and engineers use this key information, in the early stages of the exploration and development cycle, for effective reservoir management and field development planning to maximize hydrocarbon recovery.

?This article describes the applications of wireline formation testers (WFTs) for pressure transient testing, fluid identification and collect fluid samples.

?Wireline Formation Testing

Wireline formation tests are performed mostly in open hole using cable-operated formation tester tool anchored at depth while a reservoir communication is established through one or more probes or/and straddle packers.

Most of the WFTs are built as a string of modules selected according to the design of the test with each module performing a specific function such as: electric, hydraulic, pump out, fluid analyzer, sampler, multi-sampler, single probe, dual probe and straddle packer. For that reason, design and implementation of wireline formation testing program should be planned carefully and define clearly objectives is very importance during this step. A good testing design should take account reservoir characteristics, type of well, drilling fluid, hole size and among others parameters depending on how complex is the reservoir and the well drilled. Figure 1 shows a typical wireline formation tester configuration for pressure testing and fluid sampling.

Figure 1: Typical Wireline Formation Tester configuration for Pressure Testing and Fluid Sampling (Fundamentals of Formation Testing, 2006).

WFTs are used in conventional reservoirs with high and low permeability (as low as 0.1 mD) with different type of hydrocarbons from gas to heavy oil. They allow measure static formation pressures at many locations in a well to build a pressure profile versus depth and gradients, downhole fluid analysis, take representative PVT fluid samples, perform micro-fracs and estimate reservoir parameter such as: vertical and horizontal permeability, and formation damage.

Understanding the Reservoir Dynamics

Formation Pressure Profiling

The primary data obtained from formation testers are measurements of pressure versus time. Formation pressures are obtained by withdrawing a small amount of fluid to generate a short transient test, called a pretest. In fact, the formation pressure is probably the single most important measurement in petroleum engineering due to it is used to estimate reserves, dynamic reservoir parameters estimation (permeability), reservoir characterization and simulation, production monitoring, well completion and fluid characterization (phase behavior, fluid properties and PVT analysis). During the drawdown period of the pretest, drawdown mobility can be estimated; being a good qualitative indicator of the permeability and productivity nearby wellbore region, however, this value of mobility is influenced by the invasion and formation damage.

Pressure versus depth profiles, mainly in virgin reservoirs, are used to determine the in-situ fluid density, fluid contacts and free water level. It is possible because the reservoir pressures are not affected by production and the equilibrium conditions that were established in the course of geologic time prevail. Additionally, vertical pressures profiles can be correlated with open hole logs of other wells to describe vertical and lateral communications. Figure 2 shows an example of an application of vertical pressure profile in an exploratory well.

Figure 2: Vertical Pressures Profiles obtained from Wireline Testers in a new well (Fundamentals of Formation Testing, 2006).

In development reservoirs, due to the influence of production, differential pressure depletion is frequently occurred and pressure gradient to estimate in-situ fluid density is not possible. However, pressure-depth profiles help to understand reservoirs dynamics behavior such as: fluid movement within the reservoir, vertical and horizontals barriers, monitor flood performance and reservoir compartmentalization. Figure 3 shows a vertical pressure profile in a well drilled in a development reservoir. This well was completed from an interval perforated in zone 1. On the pressure profiles taken some time after initial completion, it is clear that in zone 1, pressure have been depleted by fluid withdrawal. The pressures in zones 2, 3, 4 and 5, with were left unperforated, nevertheless have been affected by vertical flow through the reservoir.

Figure 3: The Pressures Profiles in a well drilled in a field development highlights low-permeability barriers and vertical flow in the reservoir (Fundamentals of Formation Testing, 2006).

Downhole Fluid Identification and Sampling

Engineers need to know accurate fluid properties for reservoir evaluation, field development plan, reserves calculations, well completion, flow assurance, inflow performance, production forecast and optimization, reservoir simulation, recovery estimation and many others applications. Collecting representative fluid samples is a need during the life cycle of a reservoir, especially in an early stage. Sampling can be performed several ways in the field: at the surface or downhole during a well test, with a wireline formation tester in open holes (in few cases in cased hole as well), or using a production fluid sampler in production wells.

The main advantage of wireline sampling is to collect multiple fluid samples at different depth from the same reservoir in an environmentally friendly and cost effective manner. All the samples can be stored in pressurized bottles and then sent to laboratory for further detailed analysis.

WFTs use a pump out module to flow reservoir fluid into and through the tool to the wellbore, and this enable reduction of filtrate contamination to obtain nearby clean native reservoir fluid. The process to monitor the fluid during the cleanup period is done using a module called fluid analyzer.

The fluid analyzer module uses mainly optical absorptions as basic method to identify the type of fluid and also uses other devices in order to make a complete understanding of the reservoir fluid in real time. The fluid analyzer could provide the following measurements: fluid identification (gas, oil, and water), hydrocarbon fluid composition, gas/oil ratio, CO2 content, fluid color, pH. live fluid density and viscosity, fluorescence, reflectance and resistivity.

In addition to fluid sampling, the data from fluid analyzer is used for advanced reservoir dynamics interpretations integrated with pressure gradients and open holes logs, understanding fluids distribution and contacts, potential connectivity and barriers, and reservoir heterogeneities. Figure 4 shows an example using pressure gradients and the information obtained from the fluid analyzer module to understand dynamically the reservoir.

Figure 4:?Pressure gradients integrated with the information obtained from fluid analyzer module to understand dynamically the reservoir (Downhole Fluids Laboratory Oilfield Review, 2010).

Permeability Characterization?

Wireline formation testers (WFT) can evaluate vertical and horizontal permeability through multiple pressure transient tests measuring at a length scale between cores and well tests. They allow perform an Interval Pressure Transient Test (IPTT) or also called Mini-DST, mainly using the straddle packer module. Also, it is possible to perform at the same time vertical interference test (VIT) adding monitoring probes above and/or below of the straddle packer.

IPTTs are relatively modern techniques to the oil and gas industry, in comparison to the conventional well tests. The conventional Drill Sterm Tests (DSTs) have been widely used in the industry to successfully obtain average reservoir characteristics on a larger scale, and to attest the lateral extent of the reservoir if a boundary is reached during the flow test. Though this method is still reliable, it is time-consuming (taking days and even weeks to be completed) and hence, incurs significant cost to E&P operators. With the introduction of the IPTTs, equivalent well testing deliverables, under a certain range of formation parameters, can now be acquired at a considerably lower cost, within hours, and have the unique advantage of targeting multiple sands zones to understand the reservoir homogeneity and quantify the effect of thin layers that are not seen by other techniques.

Figures 5 and 6 are shown an example of IPTT. Figure 5 shows the pressure difference and derivative of pressure with respect of function of time for a buildup. Spherical and radial flows were seen, and vertical and horizontal permeability were estimated. Figure 6 shows a pressure and pump rate versus time during an IPTT and also showing the sequences to collect fluid samples at the same test.

Figure 5: Pressure Difference and Derivative of Pressure with Respect of Function of Time (Oilfield Review Characterizing Permeability using Formation Testers, 2001)

Figure 6: Pressure and Pump Rate versus Time during an IPTT (Oilfield Review Characterizing Permeability using Formation Testers, 2001)

References

Ayan, C., Hafez, H., Hurst, S., Kuchuk, F., O′Callaghan, A., Peffer, J., Pop J., Zeybek, M.:Charaterizing Permeability with Formation Testers. Oilfield Review Automn 2001.

Ayan, C., Corre P-Y., Firinu M., Garcia G., Kristensen M., O’Keefe M., Pfieffer T., Tevis C., Zappalorto L., Zeybek M.: New Dimensions in Wireline Formation Testing. Oilfield Review Spring 2013.

Creek J., Cribbs M., Dong C., Elshahawi H., Hegeman P., Mullins O., O’Keefe M., Peters K., Zou J.: Downhole Fluids Laboratory. Oilfield Review Winter 2009/2010.

Fundamentals of Formation Testing. Schlumberger. 2006.