STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Not applicable.

BACKGROUND

The invention is related to the field of instruments used to sample fluids contained in the pore spaces of earth formations. More specifically, the invention is related to methods of determining hydraulic properties of anisotropic earth formations by interpreting fluid pressure and flow rate measurements made by such instruments.

Electric wireline formation testing instruments are used to withdraw samples of fluids contained within the pore spaces of earth formations and to make measurements of fluid pressures within the earth formations. Calculations made from these pressure measurements and measurements of the withdrawal rate can be used to assist in estimating the total fluid content within a particular earth formation.

The oil and gas industry typically conducts comprehensive evaluation of underground hydrocarbon reservoirs prior to their development. Formation evaluation procedures generally involve collection of formation fluid samples for analysis of their hydrocarbon content, estimation of the formation permeability and directional uniformity, determination of the formation fluid pressure, mobility, permeability and many others. Measurements of such parameters of the geological formation are typically performed using many devices including downhole formation testing tools.

Recent formation testing tools generally comprise an elongated tubular body divided into several modules serving predetermined functions. A typical tool may have a hydraulic power module that converts electrical into hydraulic power; a telemetry module that provides electrical and data communication between the modules and an up-hole control unit; one or more probe modules collecting samples of the formation fluids; a flow control module regulating the flow of formation and other fluids in and out of the tool; and a sample collection module that may contain various size chambers for storage of the collected fluid samples. The various modules of a tool can be arranged differently depending on the specific testing application, and may further include special testing modules, such as nuclear magnetic resonance (NMR) measurement equipment.

In certain applications the tool may be attached to a drill bit for logging-while-drilling (LWD) or measurement-while drilling (MWD) purposes. Examples of such multifunctional modular formation testing tools are described in U.S. Pat. Nos. 5,934,374; 5,826,662; 4,936,139; and 4,860,581.

In several embodiments, the present invention is over the prior art as, the present invention can have: at least one probe with at least one port aperture; at least one additional aperture with a different shape or multiple apertures used to form a different effective shape; pressure measured from at least two flowing apertures is processed to determine anisotropy Kv/Kh; pressure is simultaneously measured from the non-flowing apertures gauges (i.e., monitoring apertures); pressure from at least one non-flowing monitoring aperture is processed to determine Kv, Khand S; and/or with two or more probes or a repositioning of the tool at different depths enables the determination of formation parameters such as dip angle and or two or more formation layer properties (i.e., Kvn, Khnand Sn with “n” being the layer number). Alternatively, both apertures can be flowing simultaneously and by varying the rates from either aperture Kv, Khand S is determined using the pressure measurement from both probes.

SUMMARY OF THE INVENTION

In several embodiments, the present invention discloses a new method of using the short duration pretesting to determine at least three formation properties (from at least two tests) such as the formation skin damage, permeability in at least one direction or a combination thereof (i.e., vertical, horizontal, radial, longitudinal spherical, etc.) and anisotropy. This is done by using probes of different effective shapes that have different pressure responses to at least one formation property such as anisotropy. By performing independent pressure tests with at least two probes the pressure and flow data is used to determine that property. Then by preforming at least one interference test between the probes where flow is induced from the formation from at least one of the probes and the pressures are monitored at both probes, a component of the permeability between the probes can be determined (i.e., spherical, vertical or horizontal). The formation skin damage can be determined using the probe shape dependent property such as the anisotropy and the component of permeability from the interference tests.

Formation testing normally involves analyzing pressure transients created by changing the pressure of the formation by withdrawing or injecting fluid into the formation followed by a period of pressure stabilization. The pressure transients can then be analyzed to determine one or more formation properties. The disadvantage to this method is that it can be very time consuming, inconclusive, limited to a few formation properties and operational conditions distorting the pressure transient. These issues are more pronounced when using a wireline or LWD formation tester in open-hole conditions encountered soon after drilling a formation interval. Typically, an open-hole formation tester with a single probe is used to perform a short duration test and only one property can be determined definitively, which is the spherical mobility (or spherical permeability if the viscosity is known). The spherical mobility determined will include the influence due to formation damage near the well bore characterized by the skin coefficient S. This skin coefficient can be determined if the pressure transient is adequate, but in most open-hole conditions this cannot be resolved accurately with a short duration transient. In addition, the spherical mobility determination is influenced by the anisotropy. If a second probe is used the mobility related to the direction between the probes can be determined without the skin effect. If the skin or anisotropy cannot be determined, then the actual formation spherical permeability and anisotropy cannot be determined with accuracy. A third probe could be added but this adds significantly to the testing complexity, testing time and reliability. The skin damage magnitude can range from 0 to over 10 and directly impacts the mobility and anisotropy measurements.

One of the embodiments of this invention uses two different probe aperture shapes that enable the anisotropy to be determined by comparing pressure disturbance and flow rates from both probes. Then by measuring the pressure disturbance that propagates to the second aperture, as in an interference test, the mobility in one direction is determined. An interference test can consist of flowing from one probe aperture while a second probe aperture is not flowing. An interference test can also be performed when both probe apertures can be flowing simultaneously, and the rate is varied from the either probe aperture. In both cases the pressure and flow rates are monitored from both probe apertures and pressure changes are observed when the flow rate is changed from either aperture. From the anisotropy and directional mobility results the actual spherical mobility can be determined without the skin effect. The skin magnitude can be determined using the pressure disturbance and flow rates from either probe because it is related to spherical mobility, anisotropy and skin. These properties are now determined using short duration tests where the magnitude of the pressure disturbance is used rather than the full pressure transient.

In one embodiment of the present invention, the method used in this embodiment and others involves determining the flow coefficients for both probes used for estimating the spherical mobility where the flow coefficient is a function of a formation property such as anisotropy. The probe aperture shapes are designed to create a different response function for the flow coefficients related to the property of interest such as anisotropy. The second step involves determining the flow coefficients for that property related to the direction between the probes. This enables at least one additional property to be determined such as skin from two or more tests.

In some embodiments of the present invention, the flow coefficients for probes of different shapes can be related to more than one property. These are generally geometric in nature, such as formation bed dip angle, tool borehole azimuthal angle, and distance to one or more bedding boundaries. If the tool is moved in the borehole to a new depth and/or azimuthal angle, additional measurements can be made to improve the accuracy of the properties determined using a library of probe coefficients for the test conditions encountered and regression methods. In addition, it is possible to introduce additional parameters such as multiple bedding plane layers in a formation interval, with each bed having a thickness, boundary condition, mobility, anisotropy and skin. Formation pressure measurements made along the wellbore can also be incorporated by using gradient analysis techniques that can delineate layer boundary and boundary conditions. In this embodiment, a large number of measurements is used to determine the formation interval properties using regression techniques such as error minimization, multivariant analysis and perturbation methods. Because the measurements can be made with short duration tests there is significant time savings. In addition, using simple pressure magnitudes rather than full transients simplifies the analysis while improving the accuracy.

