Measuring and testing – Borehole or drilling – Pump test
Reexamination Certificate
2000-02-08
2001-12-18
Williams, Hezron (Department: 2856)
Measuring and testing
Borehole or drilling
Pump test
C073S054330
Reexamination Certificate
active
06330826
ABSTRACT:
This invention relates to an apparatus and a method for monitoring the sag of weight material in drilling fluids, particular for field testing at a drilling rig site.
BACKGROUND OF THE INVENTION
Hydrocarbons such as oil or gas are extracted from underground reservoirs through a well bored into the rock formation by means of a drill bit connected to a drill string or coil tubing. Rock cuttings are removed from near the bit by means of a drilling fluid or mud pumped down the pipe or coil tubing, which mud returns the cuttings to the surface via the annulus between drill string and the formation. In order to avoid wellbore instability the pressure of the mud at any depth should exceed the pressure of liquid in the pore space of the formation.
Commonly the mud is densified by the addition of a finely divided weighting agent, typically barite but often other minerals, the weighting agent being of density exceeding the unweighted mud density, and added in quantity judged to achieve the proper mud density. The lower limit of the particle size of the weighting agent is selected such that it does not affect the mud's rheology, neither damage the permeability of the formation in a reservoir section. An upper limit of the particle size is given by the mesh size of the shale shaker screens used to remove cuttings from the circulated mud. In practice the size lies in the 10 micron to 100 micron range. Being of this size the particles are not suspended by Brownian motion and so are prone to sediment under the influence of gravity; this process is termed “sag”, or “barite sag”.
A sag monitor as described herein measures the sedimentation of weight agents during downhole operations where the mud is neither circulated nor in a gelled state, e.g. during tripping, logging, or placing casing.
A device with similarity to that described herein was presented by Jefferson (ASME 91-PET-3, ASME Energy Sources Technol. Conf. & Exhibition, New Orleans, USA, Jan. 20-24, 1991). In his method the mud is sheared between the outside diameter of the rotor of Fann 35 Oilfield Rheometer, and the inside diameter of a Fann heat cup with a flat bottom. After shearing for a specified time, samples of mud are extracted from the top and bottom of the mud sample in the heat cup using a syringe inserted through the annulus at the top of the device. The density of the extracted samples was measured with either a pocket mud balance, or by weighing a known volume of extracted mud on a balance.
Among the disadvantages found in the known apparatus is a non-reproducible sampling technique. Furthermore, the apparatus tends to underestimate the sag. Also, the densest mud collects in a vertically heterogeneous layer at the base of the heat cup, and is not precisely sampled by the needle inserted from the top. Similarly there is a vertical gradient of mud density in the heat cup, leading to similar errors of density in the mud taken from the top of the heat cup.
The present invention addresses the above deficiencies in Jefferson's method, and further includes a method of real-time measurement of the sedimentation process.
SUMMARY OF THE INVENTION
The invention comprises of a conically or frustro-conically shaped inner body and an outer body having an opening with contours closely matching those of the inner body. In operation the inner body is suspended within the opening of the outer body leaving a gap which defines a conically or frustro-conically volume. Inner and outer body can perform a rotational movement with respect to each other, preferably by rotating the inner body (rotor) in the opening of the stationary outer body.
The cone angle is preferably close to 90 degrees.
Preferably the volume defined by the gap between inner and outer body is constant with respect to size and shape during rotation.
In a preferred embodiment, the inner body and the matching opening are extended by a cylindrically shaped part at one end, thereby increasing the volume of the gap between inner and outer body. In a preferred variant of this embodiment, the gap width is essentially constant in both the vertical and inclined parts of the gap. The width of the gap is preferably in the range of 1 to 3 mm.
The dimensions of the volume defined by the gap between inner and outer body and the operating conditions are preferably chosen such that, on rotation of the rotor, the mud contained in the intercylinder gap is subject to laminar shear, typically at rates between 1 to 60 s
−1
. Simultaneously, it is advantageous to reduce the amount of mud necessary to make a measurement. Preferably the volume of the gap is less than 100 milliliter to facilitate the use of the sag monitor as field testing.
On sedimentation under shear the barite falls into the conical part of the gap and is focussed to the vertex. Hence mud at the vertex is strongly densified in a short time, typically 20 min. The barite (or other weighting agent) in the mud, in excess of that in the unsagged mud, is measured. This may be done via the mud density, or it may be done by other methods, e.g. from the mud's electrical or other properties. To measure the density directly, mud is extracted from the vertex with a small syringe whose entrance nozzle is as close as possible to the vertex. The mud volume extracted is limited to a fixed volume by the design of the syringe, and the mud density is obtained by weighing the full syringe.
It is feasible to extract the sample from the surface of the mud and determine the mud sag from the density of the lighter mud. However it is believed that such a measurement can be distorted by the development of bubbles in the sheared mud. The bubbles rise to the surface and make it difficult to achieve an accurate measurement of the sample's density.
The mud's electrical or other suitable properties may be measured via electrodes or sensors in the wall of the stator, so giving a continuous method of monitoring the sag. It is shown how a simple calibration may be obtained to derive the mud density from the electrical properties of the mud.
Although described in terms of a device for measuring the sedimentation of weighting agents in sheared drilling fluids, the same invention can be used for other particulate fluids used in the oilfield industry such as cement slurries or spacers, or for fluids where the particles are either denser (e.g. paints) or are less dense (e.g. oil-in-water emulsions) than the continuous or external phase.
These and other features of the invention, preferred embodiments and variants thereof, and further advantages of the invention will become appreciated and understood by those skilled in the art from the detailed description and drawings below.
REFERENCES:
patent: 3581558 (1971-06-01), Porter et al.
patent: 3744633 (1973-07-01), Schmidt, Jr. et al.
patent: 4991998 (1991-02-01), Kamino et al.
patent: 5103679 (1992-04-01), Porter et al.
patent: 5777212 (1998-07-01), Sekiguchi et al.
patent: 0 417 885 A2 (1990-06-01), None
patent: 2049521C (1995-12-01), None
Mitchell Thomas O.
Politzer Jay L.
Schlumberger Technology Corporation
Williams Hezron
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