Dual packer-probe module extends formation test capability to include directional components of in-situ permeability.

Once considered a pretest tool, formation testers have evolved to offer versatility in formation evaluation, accessing 3-D in-situ permeability data and providing critical pressure measurement. Results from a formation tester (FT)-dual packer combination used in Angola's Congo Basin indicates the degree to which formation testers have evolved, now addressing an intermediate scale of testing.

Drill Stem Testing (DST) has traditionally been used to evaluate reservoir productivity and obtain fluid samples in exploration and appraisal wells. However, DSTs can be costly and may be undesirable in some circumstances. A third-generation wireline FT is now being used to reduce or avoid the need for DSTs in marginal offshore fields.

State-of-the-art modularity and functionality present in such evolved FT tools present operators with an effective alternative, and furthermore allow them to obtain directional components of in-situ permeability past the damaged zone. This new capability represents an important advance in formation tester development in that these tools have been commonly used for obtaining vertical pressure profiles rather than mobility data; horizontal permeability data can now be obtained. The tool is used to obtain pressure transient data past the damaged zone and into virgin rock and fluid.

A combination of methods are being used to measure in-situ the variation of effective permeability with water saturation, using the fluid fractions measurement while sampling in tandem with openhole logs and interval-test data.

Key to productivity and total recovery

During a standard DST, drillers isolate an interval of the borehole and induce formation fluids to flow to surface, where flow volumes are measured before they are burned or sent to a disposal tank. The FT-dual packer module provides similar function to a DST, but on wireline and at a smaller scale. The advantages of the FT-dual packer are significantly less cost and no fluids to surface. Additional cost benefits are derived from cheaper downhole equipment, shorter operating time and the elimination of surface handling equipment.

While a full DST reveals average reservoir characteristics and assesses the initial producibility of a well, permeability variations are averaged; however, these variations are not located or quantified. With the help of logs, the smaller volume FT-dual packer can be used to more accurately evaluate key intervals. The procedure for interpreting pressure transients and software is the same for the FT-dual packer or a full DST.

While early wireline formation testers were designed primarily to obtain vertical pressure profiles and collect fluid samples, the third-generation tool has a modular design made for measuring in-situ properties of rock such as vertical and horizontal permeabilities in addition to taking pressure and single phase fluid samples. It can be configured with different modules for different applications or handle varying conditions of well and formation. The FT-dual packer module provides accurate in-situ permeability measurements and takes several fluid samples per trip (as many as 18 to 21 downhole samples) in open and cased hole to provide answers to drilling, production, reservoir engineering and geological questions.

Operated by an electrically driven hydraulic system and downhole pump, the dual-packer module can be inflated via the downhole pump by using wellbore fluid. The tool can be inflate-set and deflated about 10 to 20 times to simultaneously measure dual packer pressure, flow rate and pressure change at the observation probe in all key zones of interest during one trip. Interval packer pressure can be observed by the engineer to determine if the packer has sealed properly and if the fluid flow is adequate to obtain a propagating pressure pulse towards the reservoir. If either condition is questionable, the tool can be retracted and moved to a more suitable test depth. At surface, formation pressures and all other related measurements are recorded in real time and monitored during the operation. Once the pressure (dual packer and probe) and flow rate measurement are obtained from that station, components of permeability can be inversed.

While the dual-packer module has a flowing area 2,000 times larger than that of a conventional repeat formation tester (RFT) probe-type tool, flow geometry measured as part of a packed cylinder more closely resembles the conventional DST well test. RFT pressure measurements are strongly affected by near-wellbore phenomena such as wellbore fluid invasion. Merging dual-packer and probe (RFT) measurement capability into one tool provides 3-D well test data from which inverse vertical and horizontal permeability as well as skin in-situ condition can be derived.

Drawdown and build-up sequences typically last for 11/2 hours or less, and the radii of investigation is proven up to 131 ft (40 m).

Understanding reservoir dynamics - interference

The openhole wireline FT-dual packer can be utilized in cased hole for purposes such as understanding well-to-well interference to convert a proposed plugged and abandoned well into an injector. The FT-packer is used to measure valuable pressure as well as reservoir characteristics to understand well-to-well communication.

During the development phase of one marginal field offshore Angola, appraisal Well 2/A was drilled with the main objective of testing for possible oil in two reservoirs that consist of sandy-dolomite and dolomite with a fracture system created by dissolution of old fractures and shells.

While the formation showed generally very poor fluid mobility, further log analysis merged with the final results from the formation tester combination indicated that the second reservoir was saturated with water and it was decided to case the well.

Analytical and simulation studies were performed and it was decided to suspend production from Well 3/A to interfere (drawdown) the pressure in Well 2/A. A pressure drop of 0 to 50 psi in Well 2/A would indicate well-to-well communication. The FT-dual packer combination was run over a 1-ft (.3-m) interval perforated with high shot density, 41/2-in. charges, with on-depth perforating being a key element to successful measurement.

The FT-packer was stationed in 7-in. casing to measure formation pressure. Packer inflation across the perforated interval took around 17 minutes, then three drawdown period tests were conducted. Measured pressure was 3539.50 psi, indicating a 35-psi pressure drop - enough to conclude well-to-well interference or communication.

Estimating productivity

In another well in the same field, openhole wireline formation testing was a key source of obtaining reservoir pressure and understanding reservoir delineation. During the well evaluation period on Well 2/B drilled as a producer, an openhole electric log and a conventional wireline FT run was run across the two target reservoirs. As part of the evaluation program, the FT-dual packer was used to perform three stations in key formation layers.

Diagnostic plots indicated spherical flow. Because the FT-dual packer production creates pressure propagations towards the reservoir and this is similar to the limited-entry or partial penetrating model of conventional welltest, interpreters needed to identify flow regimes during the build-up to calculate flow geometry inversed directional permeability. In a homogeneous layer, these regimes are early radial flow around the dual interval, pseudo-spherical flow until the pressure pulse (propagation) reaches a boundary, and total radial flow between upper and lower no-flow boundaries.

Spherical permeability is estimated from a log-log plot of pressure derivative versus a particular function of time. A type curve match for pressure difference and its derivative was generated to obtain horizontal permeability, vertical permeability and skin. The interesting observation after analysis of vertical permeability data (which was significantly larger than the horizontal permeability) along with the anisotropy ratio suggested that reservoir B layers were supported by vertical hairline fractures.

Data obtained from the FT-Dual Packer-Probe station was used later to derive the production index [(PI) or the volume a well can produce per unit pressure drop] on Well 2/B. Because the data indicated a PI five times lower than expected, it was concluded that the investigated volume of rock during FT-Dual Packer test had poor reservoir quality when compared to the volume sampled during the average long-term DST.

Better field development strategies

Permeability is difficult to determine as it often changes dramatically with scale and direction. The latest generation of wireline formation testers, equipped with packers and multiple probes, now provide cost-effective, in-situ directional permeability information (particularly in the intermediate range between core and DST testing) not previously available with other techniques or with former generations of wireline formation testers. Use of such variations in formation testers as the FT-dual packer module is expanding the use of modern testers to determine permeability and help make important well-completion and reservoir-management decisions.