Downhole drilling for oil and gas typically involves an operator drilling a well by rotating a drillstring having a drill bit attached, and using the system to bore through a formation. In a common form of drilling called rotary drilling, a rotary table or a top drive rotates the drillstring. A bottom hole assembly (BHA) on the drillstring has increased weight and provides necessary force on the assembly's drill bit to optimize the rate of penetration (ROP) of the system while drilling through the formation, thereby increasing drilling efficiency.
In measurement while drilling (MWD) applications, downhole sensors are disposed along the body of the drillstring and are used for measuring geophysical properties of the surrounding wellbore during drilling operations. During MWD operations, it is also commonly desired for the operator to take periodic measurements downhole using the downhole sensors, at particular sampling rates (i.e., based on timing) per the distance drilled in the wellbore. However, these sampling rates may vary, requiring many measurements per drilled foot within the wellbore or requiring fewer measurements per drilled foot.
If the ROP of the drill bit changes, however, and if the downhole sensors are unable to change sampling rates based on the changed ROP, then the downhole sensors may take too few measurements (e.g., if the ROP increases to a rate faster than the sensors had been originally set to take measurements), or the sensors may take measurements too often (e.g., if the ROP decreases). Occasionally, the formation that the drill bit bores through downhole may be very dense, causing the ROP to decrease to the point that the sensors take redundant measurements (i.e., the sensors actually take multiple snapshots of the same data). Therefore, if the downhole sensors are unaware of the drilling depth or the rate that the drilling depth changes (i.e., the ROP), the sensors may take multiple, repetitive measurements unnecessarily downhole. This leads to needless power consumption and memory storage capacity issues downhole.
Different forms of data can be useful downhole during operations. For example, information related to time, depth, and/or ROP could be useful for operating the downhole sensors and other components of the downhole assembly. For example, other devices downhole, such as rotary steerable tools, directional drilling equipment, etc., can also benefit from different forms of information, such as incremental depth or real-time depth.
It is possible to transmit data downhole to the BHA by using pressure communication, electromagnetic communication, classical RPM communication, and other forms of known telemetry.
As one example, U.S. Pat. No. 4,763,258 discloses how information can be communicated to a microprocessor downhole via an inclinometer and magnetometer by selectively rotating a drillstring during a data time interval through a predetermined magnitude of angular displacement or angular velocity. In another example, U.S. Pat. No. 6,267,185 discloses how a drillstring can be rotated at surface sequentially through one or more discrete angles of rotation or at different angular rates to generate a command code. The sequence of discrete angular rotations or angular rates is sensed downhole by a gyroscope and decoded as a command in a microprocessor, which transmits the decoded command to controlled equipment.
The various telemetry techniques typically require special equipment, may be incapable of being performed while drilling a borehole, and may adversely affect operations. Depending upon the technique, for example, many of these solutions may be very cumbersome, may be manual in nature, and may require particular transmission windows of time for communicating encoded data. Due to these issues with known solutions in the art, as well as the slow rates of data transmission and the disruptive nature these solutions contribute to drilling operations, it is impractical to use these solutions to transmit useful data (e.g., time, depth, and/or ROP information) to the BHA.
It is therefore desirable to have a system and method for communicating the incremental depth and other useful data of a borehole tool to downhole sensors while remaining transparent to drilling operations. The subject matter of the present disclosure is directed to overcoming, or at least reducing the deficiencies of, one or more of the problems set forth above.