Wells – Processes – With indicating – testing – measuring or locating
Reexamination Certificate
2000-07-19
2001-10-02
Suchfield, George (Department: 3672)
Wells
Processes
With indicating, testing, measuring or locating
C073S152360, C073S152380, C166S250170, C175S050000, C175S230000
Reexamination Certificate
active
06296056
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of well drilling and completion. More specifically, the present invention relates to direct measurement apparatus and methods for evaluating subsurface conditions in a wellbore.
2. Description of the Background Art
In a typical well drilling operation, conditions in the wellbore must be closely monitored and controlled to optimize the well operation and to maintain control of the well. One of the most important conditions in well drilling procedures is the bottomhole pressure of the circulating drilling fluid or “mud” used in forming or conditioning the well. The actual or effective density of the mud is an important condition that can be affected by a number of different variables related to the composition of the mud, the characteristics of the formation being penetrated by the wellbore, the dynamics of the drilling mechanism, and the procedures being implemented in the wellbore. In this latter regard, for example, the circulation of the fluid creates an effective density within the wellbore, referred to as an equivalent circulating density, that exceeds the static density of the fluid. The equivalent circulating density is caused by pressure losses in the annulus between the drilling assembly and the wellbore and is strongly dependent on the annular geometry, mud hydraulics, and flow properties of the well fluid. The maximum equivalent circulating density is always at the drill bit, and pressures of more than 100 psi above the static mud weight may occur in long, extended reach and horizontal wells.
This equivalent circulating density, which must be known in order to determine well pressures existing at different locations within the wellbore, may be calculated using hydraulics models from input well geometry, mud density, mud rheology, and flow properties, through each component of the circulating system. There are, however, often large discrepancies between the measured and calculated pressures due to uncertainties in the calculations through poor knowledge of pressure losses through certain components of the circulation system, changes in the mud density and rheology with temperature and pressure, and/or poor application of hydraulics models for different mud systems.
In many high pressure, high temperature (HPHT), deepwater, and extended reach and horizontal wells, the margin between the formation pore or collapse pressure and the formation fracture pressure often diminishes to the point that the equivalent circulating density can become critical. In extreme cases, the well may flow or cave in while the pumps used to circulate the mud are off (“pumps off”), allowing the well fluid to flow into the formation. Accurate determination of the actual static and dynamic mud pressures within the wellbore is therefore a critical design parameter for the successful drilling of these wells.
Another phenomenon affecting pressures in the wellbore results from movement of the drill string. As the drill string is lowered into the well, mud flows up the annulus between the string and the wellbore and is forced out of the flowline at the well surface. A surge pressure results from this movement, producing a higher effective mud weight that has the potential to fracture the formation. A swabbing pressure occurs when the pipe is pulled from the well, causing mud to flow down the annulus to fill the void left by the pipe. The pressure effectively reduces the mud weight and presents the potential for inducing a discharge of fluid from the formation into the wellbore. As with the equivalent circulating density measurements, the swab and surge pressures are strongly dependent on the running speed, pipe geometry, and mud rheology involved in the drilling or completion of the well. These pressures reach a maximum value around the bottom hole assembly (BHA), where the annular volume between the drilling string assembly and the surrounding wellbore is the lowest, and thus where flow through the well is the fastest.
Theoretical and experimental evidence suggests that during running pipe in and out of the wellbore, a much larger pressure differential is exerted on the formation than is experienced from static and circulating pressures during drilling, unless the pipe running speed is lowered significantly. Formation susceptibility to wellbore instability, although not problematic while drilling, may increase due to the swab and surge pressures incurred during tripping when the entire pipe string is rapidly withdrawn or reinserted in the well.
Modeling swab and surge pressure is difficult because of the manner in which the fluid flows as the pipe is moved within the well. A moving pipe causes the mud adjacent to the pipe to be dragged with it to a certain extent, although the bulk of the annular fluid is moving in the opposite direction. The mechanics are therefore different from the hydraulics calculations described for the mud circulation since, in that case, fluid flow is considered to be only moving in one direction. Swab and surge hydraulics models therefore require a “clinging constant” to account for the two relative motions.
A pressure surge caused by breaking the gels when increasing the flow rate too quickly after breaking circulation has been responsible for many packoff and lost circulation incidents. In this situation, where the well circulation is terminated for a period of time (“pumps off”) and then reinitiated (“pumps on”), if the circulation rate is reinitiated too quickly, a pressure surge is created in the mud, causing a damaging imbalance with the formation. This danger, which is particularly evident in high angle wells, led to the procedure of slowly bringing the volume of the mud pumps up anytime after circulation is temporarily suspended. A pressure surge associated with restarting circulation may also be caused by a restriction in the annulus due to cuttings sagging and accumulating while the mud is static.
In extended reach and horizontal wells, hole cleaning can become critical. If parts of the wellbore are unstable, as in common in these types of wells, the accumulation of cuttings, beds, and an overloaded annulus make it difficult to clean the hole properly. Remedial measures, such as control drilling, the pumping of viscous pills, and wiper trips, are commonly employed in an attempt to avoid packing off and sticking the pipe. These procedures, however, consume valuable time and may also damage the formation leading to further wellbore instabilities.
Yet another situation where knowledge about the subsurface conditions is important occurs when drilling out of the bottom of a casing shoe into new formation. It is common to perform a leak-off test (LOT) to determine the strength of the cement bond around the casing shoe. However, because of the small margins between the formation pore or collapse pressure and fracture pressure in many wells, the LOT has become a critical measure of the formation strength and is used as a guide to the maximum allowable circulating pressure that may be used in a subsequent hole section without breaking down the formation and losing circulation in the well.
Conventionally, LOT pressures are recorded at the surface of the well. The measurements must be corrected for the pressure being exerted by the mud column. To obtain an accurate reading in these surface conducted measurement procedures, the mud must be circulated thoroughly to condition it to produce an exact and even density for the LOT calculation. This process can be time-consuming, and the calculated results are subject to the correctness of the information and assumptions used for the values of the variable conditions affecting the mud column density.
Subsurface pressure information is especially important when the well “takes a kick” during drilling. The term “kick” is commonly employed to describe the introduction of formation gas, a lower density formation fluid, or a pressured formation fluid into the wellbore. If not controlled, the kick can reduce the density of the drilling fluid s
Browning & Bushman P.C.
Dresser Industries Inc.
Suchfield George
LandOfFree
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