Hydraulic calibration of equivalent density

Details Hydraulic calibration of equivalent density Hydraulic calibration of equivalent density

1999-11-23

2002-07-30

Lefkowitz, Edward (Department: 2862)

Data processing: measuring, calibrating, or testing

Measurement system in a specific environment

Earth science

C703S006000

Reexamination Certificate

active

06427125

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to oil well drilling and, in particular, to more efficient calibration of equivalent circulating density (ECD) and other hydraulic measurements.
2. Description of the Related Art
In the development, completion, and operation of natural hydrocarbon (e.g. oil) reservoirs, various telemetric systems and techniques are employed to make downhole measurements readily available at the surface in real-time. In particular, MWD (measurements-while-drilling) and LWD (logging-while-drilling) techniques include any type of data transmission during drilling from sensor or detector units located within the well borehole. The borehole sensors may be located in the drill bit, in the bottom hole (or borehole) assembly (BHA), in the drill string above the mud motor, or in any other part of the sub-surface drill string. Present MWD/LWD telemetry systems employ drilling fluid or mud pulse telemetry, electromagnetic telemetry, or acoustic telemetry through the drill string itself, to transmit sensed data to the surface, and remain limited in bandwidth (data bit rates are typically in the 1 KHz range or lower).
Oil and gas wells are typically drilled with circulating drilling fluid systems. In such a system, drilling fluid, or “mud”, is pumped from a reservoir at the surface of the earth down through the hollow drill string such that it exits the drill string at the drill bit and returns to the surface by way of the annulus between the borehole and the drill string. The drilling mud serves to maintain hydrostatic pressure within the borehole so that the internal pressure of formations penetrated by the bit is controlled, to provide a means of removing cuttings from the borehole and of conveying these cuttings to the surface of the earth. The drilling mud also serves to cool and lubricate the drill bit.
In mud pulse telemetry techniques, data from the downhole sensors is transmitted by means of a mud pressure pulse generator, which is part of the drill string. The generator generates pressure pulses in the drilling fluid or mud column, typically by way of a valve or siren type of device. This can only be done when there is a sufficient mud flow-rate (Q), when the pumps, which drive a circulating mud fluid, are on. Suitable generators used with MWD techniques are, for example, described in U.S. Pat. Nos. 4,785,300; 4,847,815; 4,825,421; 4,839,870 and 5,073,877.
The pulses are detected at the surface by suitable means, e.g., pressure sensors, strain gauges, accelerometers, and the like, which are usually directly attached to the drill string or the standpipe. Data may be transmitted to receivers and processors at the surface via alternate techniques as well, such as wireline tools via hard wired cables which contain electrical and/or fiber optic conductors which relay data to the surface (the wireline tools function would typically involve communicating with the nearby downhole MWD or LWD tools based on inductive coupling or other principles). Data transmission rates of conventional mud pulse telemetry systems are very low, e.g. 3 to 6 bits/sec, which is much lower than that of wireline systems.
One type of MWD measurement is annular pressure while drilling (APWD), which provides a downhole pressure measurement. In APWD, an annular sensor is provided that measures downhole annular pressure, and typically also temperature. These data readings or measurements are transmitted to the surface, e.g. by mud pulse telemetry. At the surface, a processor may be used to analyze the pressure data. When pressure is monitored in the context of other drilling parameters and in view of hydraulics principles, it is possible to identify undesirable drilling conditions, suggest remedial procedures, and help prevent serious problems from developing. Obtaining real-time downhole annular pressure information can be especially desirable in extended reach wells, high pressure/high temperature (HPHT) wells, in slim wells, and in deep water environments where large flowing frictional pressure losses or very narrow pressure margins can exist.
APWD pressure measurements can be also used to determine equivalent mud density, another useful downhole measurement. Equivalent density is typically referred to as equivalent circulating density (ECD), which is technically the equivalent mud density when the mud is circulating. When the mud is not circulating, equivalent density is referred to as equivalent static density (ESD). ECD is often used as a general term to encompass both ECD and ESD, and is an important parameter which represents the integrated measure of the fluid behavior in the annulus.
ECD is computed by dividing the measured pressure by true vertical depth (TVD), which is known at the surface. The ECD computed based on a given APWD pressure measurement may be referred to as an ECD measurement or measured ECD. If the measured CD is too high or too low, in comparison to some expected ECD, corrective or other responsive steps may be taken, to try to maintain the ECD within a desired range. For example, a higher ECD can indicate that cuttings are not being cleaned efficiently, and a lower ECD may indicate that a gas influx has occurred. Thus, it is useful to know ECD because it can help prevent costly drilling problems (mostly related to poor hole cleaning) and can aid in positively identifying kicks, inflows, and other events which can lead to unsafe drilling conditions.
Thus, by measuring pressure and determining ECD from this measurement, and by comparing this ECD measure to some baseline or expected ECD measure, corrective steps can be taken to maintain ECD within a desired range. This can help prevent lost circulation and maintain borehole integrity (including managing swab, surge, and gel breakdown effects). Similarly, it is also useful to monitor the downhole pressure measurements.
Hydraulic-related measurements such as downhole APWD pressure measurements and measurements derived therefrom, such as equivalent density, may be referred to generally as hydraulic measurements. In addition to ECD and downhole pressure, it is also useful to measure and monitor other hydraulic measurements, such as standpipe pressure, internal pressure, and Turbine RPM (TRPM). TRPM is the RPM of a downhole turbine that generates electricity as mud flows therethrough. This electricity is often used to power downhole tools. Internal pressure is the pressure inside the drillpipe, and is typically measured by an Internal Pressure While Drilling (IPWD) sensor, for the purpose of detecting drill-pipe leaks and their position. An IPWD sensor is typically identical to an APWD sensor, but instead of being in the annulus it is inside the drillpipe.
Hydraulic measurements such as downhole pressure and ECD, however, are sensitive to a variety of events and factors. Thus, in order to diagnose events and analyze real-time hydraulic measurements which are taken under certain prevailing conditions, there is a need to account for as many of the factors as possible. Under current technology, sophisticated modeling or simulation is not sufficient for such analysis, because some of the factors that affect pressure measurements cannot be easily predicted or modeled. Factors which affect pressure measurements include mud properties (including changes related to pressure and temperature), flow-rate, flow-regime, drill-string rotations per minute (RPM), drill-pipe eccentricity, and hole geometry (size and shape).
Both RPM and the flow-rate are known at the surface. However, because the other factors that affect the pressure measurement and thus the nominal ECD calculation are unpredictable and not always known or knowable at the surface, there is a need to calibrate the hydraulic measurement in-situ, i.e. to establish a base which indicates what the hydraulic measurement (e.g., downhole pressure or ECD) should be for a given flow-rate Q and drill-string RPM. The terms “ECD Calibration”, “Hydraulic Calibration”, and “Hydraulic Fingerprinting” are commonly used to describe such a calibr

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