I am going to take a small break from the continuing series of blog entries on economics and risk analysis to address a specific technology. This is in response to multiple requests, so if there is a topic you want to address, please don’t hesitate to contact me.
Wireline and LWD formation testing while drilling has numerous advantages over drill stem testing and conventional testing of completed wells. In this and the following two blog entries we will discuss these powerful tools. In this entry I will give an overview of the technology and discuss the theory for interpreting permeability and other pressure transient analysis derived properties. In the next blog entry I will go over some of the extremely powerful techniques available to evaluate hydrodynamic parameters including identification of in-situ fluid gradients, identifying the location of fluid contacts and determining if any two formation intervals (in a single well or separate wells) are in hydrodynamic equilibrium. There are many more uses of these tools including during the course of depletion, optimizing waterfloods, and in hydraulic fracturing diagnostics.
Reservoir evaluation during the drilling and completion process involves many tools including mud logs, Logging While Drilling (LWD), open hole logs, coring, etc. Estimates of reservoir permeability and productivity along with pore fluid contents usually require some sort of flow test to achieve a desired level of accuracy. Most of the pressure transient analysis methods discussed so far are applied to completed wells or to a conventional drill stem test (DST).
One of the most powerful well testing tools in newly drilled wells is the formation tester (FT) which allows operators enormous flexibility during the drilling of a well. It can be conveyed on wireline or on the drillstring using advanced LWD technology. The latter is particularly important when it is difficult to convey the tool to the desired depth using wireline such as in steep directional wells, horizontal laterals or in difficult downhole environments. Wireline FT tools may be deployed in such difficult downhole environments using pipe-conveyed technologies; however, advances in LWD formation testing such as Baker Hughes’ TesTrak™ LWD formation-pressure testing service have offered operators greater flexibility.
FT devices offer the operator a chance to measure pressures at many locations in a well accurately and efficiently. They can be verified by repeat measurements and are valid over a wide range of mobilities. With advanced pressure transient techniques, directional permeabilities can be measured quantitatively offering improved reservoir characterization and the ability to correlate petrophysical properties with permeability and productivity measures.
Example of Formation Tester (Reservoir Characterization Instrument (RCI), courtesy Baker Hughes)
FT devices offer the operator the chance to recover representative formation fluids captured and maintained above saturation pressures, preserving concentrations of non-hydrocarbon diluents such as H2S and CO2 with minimal contamination. Advances in downhole measurements allow rapid estimation of in-situ PVT properties including density, viscosity, GOR, FVF, bubble point pressure, sulfur content and compressibility along with fluid typing and identifying drilling fluid contamination. Other advanced applications include Mini-DSTs and determination of parameters vital to hydraulic fracturing using Micro-Fracs.
Formation testers use one or more snorkels that are pushed flush into the formation face at a desired depth by backup arms on the opposite side of the tool. They may or may not have straddle packers adjacent to them to provide isolation. The snorkel can allow a small amount of fluids into the tool for pressure identification or larger volumes of fluids that can allow clean samples of reservoir fluids to be analyzed and/or recovered to surface. In either case, high resolution quartz crystal pressure gauges are used for accurate transient testing and pressure measurements.
The following figure shows the conceptual design of such a tool along with the measured pressures as a function of time.
Most of the pressure transient analysis techniques discussed so far have been applicable for production or injection wells that have been completed and “cleaned up.” DSTs and FTs require the reservoir engineer to ensure that the correct reservoir conditions are being identified. Invasion of mud filtrate during the drilling process may cause an excessively high pressure in the near wellbore region called “supercharging.” It occurs in environments where the mudcake does not adequately isolate the wellbore pressure from the formation. The supercharging is a bigger problem in LWD environment where active mud circulation limits filtrate cake growth. As a result, the leak-off rates are higher in dynamic mud conditions in LWD than in wireline where mud condition is static. Supercharging is larger if mud cake permeability Km is high and formation permeability Kf is low. It is commonly observed both with wireline and LWD measurements if the permeability of the formation is less than 1 mD.
Flow towards a single point in a reservoir typically results in spherical flow. In a layered reservoir this may give way over time to cylindrical flow. In an FT, flow is restricted due to the probe and results in semi-hemispherical flow.
Various analysis techniques have been borrowed from the conventional well testing studies to analyze the FT derived data. The “Drawdown Mobility” calculation incorporates the pressure drawdown corresponding to the piston drawdown rate in Darcy’s equation to calculate the near wellbore mobility.
Sgeom : Flow geometry effect due to hemi-spherical flow
kdd : Drawdown permeability
qdd : Drawdown piston rate
rp : Probe radius
rw : Wellbore radius
µ : Viscosity
There is a drawback in this analysis particularly in very low permeability formations where transitional behavior of pressure with respect to the piston rate becomes more significant due to tool storage effect. In low permeability formations the flow rate from the formation can be different from the piston drawdown rate.
Mobility calculation by Formation Rate Analysis (FRA)1,2,3,4 accounts for the tool storage effect by calculating the system compressibility during the drawdown within the fixed tool volume. In FRA, the formation flow
The subscripts for flow rate q represent accumulation, formation flow and piston drawdown and implicitly assume a small density variation. D’arcy’s law for this system becomes
Go : Geometric factor (4.67)
rp : Probe radius (in cm)
k : Permeability, md
µ : Viscosity, cp
Adding the mass balance with respect to time and liquid accumulation rate
We can then solve for pressure as a function of time as follows:
A plot of P(t) vs. formation rate should approach a straight line with negative slope and intercept P* at the P(t) axis and the mobility is calculated from the slope. The FRA plot should yield identical slopes for both buildup and drawdown if there was constant compressibility. Compressibility effects can be resolved using multi-linear regression techniques.
Thanks to Sefer Coskun, Reservoir Engineering Manager with Baker Hughes RDS in the Global Geoscience group for assistance with this and the two following blog posts.