1. Field of the Invention
This invention generally relates to the testing of subterranean formations or reservoirs, and more particularly to an apparatus and method of acquiring highly accurate formation pressure information while drilling a well.
2. Description of the Related Art
To obtain hydrocarbons such as oil and gas, well boreholes are drilled by rotating a drill bit attached at a drill string end. The drill string may be a jointed rotatable pipe or a coiled tube. Boreholes may be drilled vertically, but directional drilling systems are often used for drilling boreholes deviated from vertical and/or horizontal boreholes to increase the hydrocarbon production. Modern directional drilling systems generally employ a drill string having a bottomhole assembly (BHA) and a drill bit at an end thereof that is rotated by a drill motor (mud motor) and/or the drill string. A number of downhole devices placed in close proximity to the drill bit measure certain downhole operating parameters associated with the drill string. Such devices typically include sensors for measuring downhole temperature and pressure, tool azimuth, tool inclination. Also used are measuring devices such as a resistivity-measuring device to determine the presence of hydrocarbons and water. Additional downhole instruments, known as measurement-while-drilling (MWD) or logging-while-drilling (LWD) tools, are frequently attached to the drill string to determine formation geology and formation fluid conditions during the drilling operations.
Boreholes are usually drilled along predetermined paths and proceed through various formations. A drilling operator typically controls the surface-controlled drilling parameters during drilling operations. These parameters include weight on bit, drilling fluid flow through the drill pipe, drill string rotational speed (r.p.m. of the surface motor coupled to the drill pipe) and the density and viscosity of the drilling fluid. The downhole operating conditions continually change and the operator must react to such changes and adjust the surface-controlled parameters to properly control the drilling operations. For drilling a borehole in a virgin region, the operator typically relies on seismic survey plots, which provide a macro picture of the subsurface formations and a pre-planned borehole path. For drilling multiple boreholes in the same formation, the operator may also have information about the previously drilled boreholes in the same formation.
Typically, the information provided to the operator during drilling includes borehole pressure, temperature, and drilling parameters such as WOB, rotational speed of the drill bit and/or the drill string, and the drilling fluid flow rate. In some cases, the drilling operator is also provided selected information about the bottomhole assembly condition (parameters), such as torque, mud motor differential pressure, torque, bit bounce and whirl, etc.
The downhole sensor data are typically processed downhole to some extent and telemetered uphole by sending a signal through the drill string or by transmitting pressure pulses through the circulating drilling fluid, i.e. mud-pulse telemetry.
Various types of drilling fluids are used to facilitate the drilling process and to maintain a desired hydrostatic pressure in the borehole. Pressurized drilling fluid (commonly known as thexe2x80x9cmudxe2x80x9d orxe2x80x9cdrilling mudxe2x80x9d) is pumped into a drill pipe through a central bore to rotate the drill motor and to provide lubrication to various members of the drill string including the drill bit. The drill pipe is rotated by a prime mover, such as a motor, to facilitate directional drilling and to drill vertical boreholes. The drill bit is typically coupled to a bearing assembly having a drive shaft which in turn rotates the drill bit attached thereto. Radial and axial bearings in the bearing assembly provide support to the drill bit against these radial and axial forces.
The drilling mud is mixed with additives at the surface to protect downhole components from corrosion, and to maintain a specified density. The mud density is manipulated based on the known or expected formation pressure. The mud in the borehole annulus is typically maintained at a pressure slightly higher than the surrounding formation. The mud may invade the formation causing contamination of the hydrocarbons or it may damage the formation if the mud pressure is too high. If the mud is maintained at a pressure too low for the surrounding formation, the formation fluid may flow into the annulus causing a pressurexe2x80x9ckickxe2x80x9d. Neither result is desirable when drilling a well.
Formation testing tools may be Formation Testing While Drilling (FTWD) tools conveyed ito a borehole on a drill string as described above or a formation testing tool may be conveyed into a borehole on a wireline. A typical wireline tool is lowered into a well using an armored cable that includes electrical conductors for transferring data and power to and from the tool. A wireline tool is typically lowered to a predetermined depth, and measurements are taken as the tool is withdrawn from the well.
Wireline and FTWD tools are used for monitoring formation pressures, obtaining formation fluid samples and for predicting reservoir performance. Such formation testing tools typically contain an elongated body having an inflatable packer, a pad seal or both sealingly urged against a zone of interest in a well borehole to collect formation fluid samples in storage chambers placed in the tool.
Resistivity measurements, downhole pressure and temperature measurements, and optical analysis of the formation fluids have been used to identify the type of formation fluid, i.e., to differentiate between oil, water and gas present in the formation fluid and to determine the bubble point pressure of the fluids. The information obtained from one or more pressure sensors and temperature sensors, resistivity measurements and optical analysis is utilized to control parameters such as drawdown rate, i.e. the rate at which tool pressure is lowered, so as to maintain the drawdown pressure, i.e. the tool pressure during testing or sampling, above the bubble point and to determine when to collect the fluid samples downhole.
Formation temperature varies based on the depth and pressure at a given point, and circulating drilling fluid tends to provide a relatively constant temperature in the borehole that is below the natural formation temperature. Circulation of fluid must be stopped whenever a wireline is being used or when a FTWD tool is used in certain sampling or test applications. Whenever circulation of the drilling fluid is stopped, the borehole temperature begins to rise. This temperature change has a temperature gradient. The temperature gradient can be quite high, thus making some instruments inaccurate.
A pressure gradient test is a test wherein multiple pressure tests are taken as a wireline or FTWD test apparatus is conveyed through a borehole. Instruments used for pressure gradient tests typically experience the constant temperature and temperature gradient conditions described above. The purpose of the test is to determine the interface or contact points between gas, oil and water. Using a typical pressure test apparatus provides approximate pressure values, that may include large error due to temperature effects. Many systems compensate for the error by utilizing complicated estimating techniques and computers to analyze the test data and determine the formation pressure at a given point. It would be desirable to have highly accurate test data to avoid the need for analytical estimations.
The present invention addresses the above-noted deficiencies and provides an apparatus and method for obtaining highly accurate pressure measurements of a formation for better control of drilling fluid hydrostatic pressure and for alleviating need for estimating formation pressure when using wireline and FTWD tools.
A Formation Testing While Drilling (FTWD) apparatus and a method are provided for obtaining highly accurate pressure measurements in a well borehole using a combination of an absolute and a differential pressure sensor for obtaining absolute pressure measurements under high temperature gradients. A high accuracy quartz absolute pressure sensor is used during a period of constant temperature. A sensor output defines a start range for a differential sensor, which has less absolute accuracy but is less susceptible to temperature effects of high temperature gradients.
The present invention uses a strain gauge, piezo resistive or similar pressure measurement system with a high resolution and good temperature compensation for a dynamic pressure measurement as a differential pressure measurement referenced to annulus pressure. A smaller full scale range pressure measurement gauge is then utilized resulting in better resolution. The strain gauge or similar system has the advantage of better temperature compensation compared to a high resolution quartz gauge used for absolute pressure measurements. However, to achieve an absolute pressure, a quartz gauge is needed to measure the absolute annulus pressure and then the differential pressure is added to it. This method has the advantage to measure very accurately the absolute pressure with the quartz gauge at constant temperature situations e.g. before mud circulation is stopped. The value is used for adjusting the initial annulus pressure setting of the differential pressure gauge measuring the draw down pressure at a temperature increase due to stopped circulation. Thus, the differential pressure gauge is used to measure the draw down pressure against annulus pressure while the quartz gauge measures the annulus pressure.
In one aspect of the present invention, a tool is provided for obtaining at least one parameter of interest such as pressure of a subterranean formation in-situ. The tool comprises a carrier member for conveying the tool into a borehole, at least one selectively extendable member mounted on the carrier member for separating the annulus into a first portion and a second portion, a first port exposed to formation fluid in the first portion, a second port exposed to a fluid containing drilling fluid in the second portion, a first sensor for determining a first value indicative of a first portion characteristic, a second sensor for determining a second value indicative of a second portion characteristic referenced to the first value. A method provided by the present invention comprises conveying a tool into a borehole, separating the annulus into a first portion and a second portion by extending at least one selectively extendable member, exposing a first port to formation fluid in the first portion, exposing a second port to fluid in the second portion, determining a first value indicative of an absolute pressure in the first portion, determining a second value indicative of a differential pressure of the second portion referenced to the absolute pressure of the first portion, and combining the first and second values using a processor, the combination being indicative of formation pressure.