Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Earth science
Reexamination Certificate
2000-05-26
2002-08-20
McElheny, Jr., Donald (Department: 2862)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Earth science
Reexamination Certificate
active
06438495
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method for predicting the direction and inclination of a drilling assembly during the process of drilling a wellbore in an earth formation and in particular to a method for predicting the direction and inclination tendencies of a drilling assembly in real-time using continuous data.
BACKGROUND OF THE INVENTION
Directional drilling is the process of directing the wellbore being drilled along a defined trajectory to a predetermined target. Deviation control during drilling is the process of keeping the wellbore contained within some prescribed limits based on the inclination angle or the deviation from the vertical of the drill bit, or both. Strong economic and environmental pressures have increased the desire for and use of directional drilling. In addition, wellbore trajectories are becoming more complex and therefore, directional drilling is being applied in situations where it has not been common in the past.
The trajectory of a wellbore is determined by the measurement of the inclination and direction (azimuth) of the drill string at various formation depths, and by a ‘survey calculation’, which represents the path between discrete points as a continuous curve. In the initial drilling of a well or in making a controlled trajectory change in wellbore trajectory, some method must be used to force the drill bit in the desired direction. Whipstocks, mud motors with bent-housings and jetting bits are used to initially force the bit in a preferred direction. New Rotary steerable systems also enable directional control while rotary drilling. All of the above bit deflection methods depend on manipulating the drill pipe (rotation and downward motion) to cause a departure of the bit in either the direction plane or the inclination plane, or both. Many terms are used in describing the directional drilling process. For the purpose of describing the directional drilling process, the following critical terms are defined:
Tool face: this can be ‘magnetic tool-face’ when referred to magnetic North, or ‘gravity tool-face’ when referred to the high side of the hole, and is the angle between the high-side of the bend and North of the high side of the hole respectively. A tool-face measurement is required to orient a whipstock, the large nozzle on a jetting bit, an eccentric stabilizer, a bent sub, or a bent housing.
Tool azimuth angle: the angle between North and the projection of the tool reference axis onto a horizontal plane, also called ‘magnetic tool face’.
Tool high-side angle: the angle between the tool reference axis and a line perpendicular to the hole axis and lying in the vertical plane.
This angle is also called the ‘gravity tool face’.
Inclination and azimuth (direction) can be measured with a magnetic single or multi-shot and a gyroscope single or multi-shot. Magnetic tools are run on a wireline, or in the drill collars while the hole is tripped or they can be dropped from the surface. Some gyroscopic tools are run on conductor cable, permitting the reading of measurements from the surface and also permitting the supplying of power down the conductor cable. Another way to measure direction, inclination and tool face is with an arrangement of magnetometers and accelerometers. Batteries, a conductor cable, or a generator powered from the circulation of the drilling mud can supply power to the tools taking these measurements. If the measurement tool is located in the bottom hole assembly (BHA) and the measurements are taken during drilling, the tool is called a measurement while drilling (MWD) tool. Details of various measurement tools, the principle of operation, the factors that affect the measurement and the necessary corrections are known to persons of ordinary skill in this technology.
The two most common MWD systems are the pressure-pulse and modulated pressure pulse transmission systems. The pressure pulse system can be further divided into positive and negative pulse systems. At the surface, the downhole signals are received by a pressure transducer and transmitted to a computer that processes and converts the data to inclination, direction and tool-face angle measurements.
Most sensor packages used in an MWD tool consist of three inclinometers (accelerometers) and three magnetometers. The tool-face angle is derived from the relationship of the hole direction to the low side of the hole, which is measured by the inclinometers. Once the readings are measured, they are encoded through a downhole electronics package into a series of binary signals that are transmitted by a series of pressure pulses or a modulated signal that is phase-shifted to indicate a logical unity or zero.
Inclination measurements at the bit can be measured during the drilling process with an ‘at-bit’ inclination (AIM) tool that is a single axis accelerometer mounted in the driveshaft of a motor. With this tool, the inclination measurement is continuously updated in both steering and rotary mode. The sensor measures the inclination of the hole at the location where the bit is currently drilling, as opposed to the inclination measurements at a section of the bottom hole assembly some distance away from the bit location, as is the case with standard MWD systems. Using the at-bit survey tool, a directional driller (DD) can initiate a steering section and see the result of steering within 5 feet, as opposed to the 50 feet or so required with a conventional MWD/LWD system. The resulting well path will be smoother and require less steering to maintain the proper trajectory. This means more rotary drilling, which in turn, means greater drilling efficiency.
Prediction of Drilling Tendency
Predicting the directional tendency of a bottom hole drilling assembly is a key element in improving the efficiency of the directional drilling process. Directional wellbores are drilled by incorporating elements into the BHA that will cause the hole to deflect in a desired manner. Stabilizers between drill collars cause a bowing action that can build, hold or drop inclination according to the placement of the stabilizers. The tendency of a BHA whilst rotary directional drilling is difficult to predict and requires years of experience for a directional driller to achieve the desired results. Steerable systems, introduced about fifteen years ago, have a bend (bent sub) in them. A positive displacement motor (PDM) turns the bit below the bend. The bend is held stationary at the desired attitude or tool face angle, resulting in wellbore curvature as drilling proceeds. Steerable system directional drilling has proven to be more practical than the rotary method. However, problems in predicting the directional tendency of both types of directional BHA's still leads to inefficiencies in the drilling process. Time is lost in tripping rotary BHA's out of the hole to alter their directional characteristics, and in slower drilling with steerable systems, where the end settings are less than optimal.
One method of predicting wellbore directional tendencies is through modeling. Finite element models attempt to represent the detailed physical interactions between the BHA and the wellbore while drilling. However, effective use of such models has been hindered by parameters that are difficult to quantify, particularly the hole gauge, the strength of the formation, and the bit anisotropy.
Prior directional tendency predictions were based on classical engineering mechanics relationships. These models often worked well, but in a limited geographic area, perhaps even one oil field, and required significant expertise. The use of steerable systems introduced stress concentrations that were more difficult to model. Further improvement in tendency predictions needed three dimensional stress models and a wider set of data for validation. The increased use of finite element programs and directional drilling databases has made more accurate tendency predictions possible, but still limited to particular geographical regions. Attempts to predict BHA tendency has slowed in recent years due to the
Chau Minh T.
Lesso, Jr. William G.
McCann Dominic P.
Rezmer-Cooper Iain M.
McElheny Jr. Donald
Schlumberger Technology Corporation
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