Sunday, January 27, 2019

Permeability Anisotropy in Horizontal Wells

Horizontal wells are excellent producers for thin reservoirs- less than 50-ft [15-m] thick - or for thicker reservoirs with good vertical permeability. The drainage pattern for an ideal horizontal well is ellipsoidal and dominated by permeability anisotropy - the half axes are related to the length of the horizontal section, the horizontal permeability and the vertical permeability. 

Vertical permeability anisotropy also affects production- the larger anisotropy, the higher the productivity index. Low vertical permeability may make horizontal wells economically unattractive. Another important issue is the horizontal permeability anisotropy. A well drilled normal to the larger horizontal permeability will be a much better producer than one drilled in an arbitrary direction or normal to the smaller horizontal permeability. Therefore, it is important to measure permeability before the horizontal section is drilled.

Most oil companies drill a vertical pilot hole to acquire as much data as possible about the target reservoir before drilling the horizontal section. These data may be used to optimize the horizontal segment length or even justify a decision not to drill the well at all.  

One method of measuring vertical permeability is to perform a limited entry drillstem test after drilling the upper 10 ft [ 3 m] of the pilot hole into the reservoir. Once the pilot hole is complete, other drillstem test may be designed to confirm the results of the first test or to provide additional data such as fluid contacts or parameter estimates of additional layers.

An alternative procedure is to use the MDT formation tester tool. Although the depth of investigation is limited to a maximum of about 33 ft [10 m] , the MDT tool has the advantage of operational efficiency. With test points carefully selected from openhole logs, the tool may provide data for subsequent well test interpretation. For example, many horizontal wells are drilled in layered reservoirs- each layer having different properties. A drillstem test conducted in the horizontal section of the well may require a layered reservoir model for analysis- a homogeneous model would lead to wrong estimates of cricital parameters such as producing length, permeability and skin factor. Parameter estimates, calculated for each layer from MDT tool data acquired in the pilot hole, would enable a layered reservoir model to be used.

Shear sonic logging measurements in the pilot hole may be used to identify the maximum and minimum horizontal stress directions. Usually, these coincide with the maximum and minimum horizontal permeability directions. A horizontal well should be drilled parallel to the direction of minimum horizontal permeability, which has the added advantage of offering the greatest borehole stability.

Once the horizontal section is drilled, it is not too late to perform drillstem tests. However, the testing equipment needs to be flexible enough to  enter the horizontal section, otherwise it has to be set in the vertical part of the well. If this happens, wellbore storage may take so much time to dissipate that the part of the pressure transient used for estimating vertical permeability and skin - early-time radial flow - is masked.

The MDT tool may also be used to measure permeability anisotropy in horizontal sections. The tool can be conveyed downhole into the horizontal section on drillpipe or by coiled tubing, and the probe orientation found using an inclinometry device. Mapping permeability variation along the well enables optimization of completion design, such as deciding where to place isolation packers or which sections of a cased hole to perforate. 

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