Differential-Sticking Mechanisms and a Simple Wellsite Test for Monitoring and Optimizing Drilling Mud Properties
- P.I. Reid (Schlumberger Cambridge Research) | G.H. Meeten (Schlumberger Cambridge Research) | P.W. Way (Schlumberger Cambridge Research) | Peter Clark (Dowell) | B.D. Chambers (BP-Amoco) | Alan Gilmour (Dowell) | M.W. Sanders (M-I Drilling Fluids)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- June 2000
- Document Type
- Journal Paper
- 97 - 104
- 2000. Society of Petroleum Engineers
- 4.3.4 Scale, 1.6.6 Directional Drilling, 1.11 Drilling Fluids and Materials, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 1.7.5 Well Control, 1.6.1 Drilling Operation Management, 1.8 Formation Damage, 1.6 Drilling Operations, 1.10 Drilling Equipment, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 2.2.3 Fluid Loss Control, 4.1.2 Separation and Treating
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Differential sticking has a major impact on drilling efficiency and well costs. It is known to be influenced by drilling-fluid properties and other parameters, such as the characteristics of rock formations. Mechanisms of differential sticking were investigated using laboratory data and information in the literature. The knowledge gained was used to design a simple and robust device for measuring the sticking properties of drilling fluids, both in the laboratory, or at the well site.
This device comprises a half-area high-pressure, high-temperature (HPHT) fluid-loss cell, modified to allow a metal sphere to be placed in contact with a growing mud filter cake. The torque required to rotate the sphere free from the mud cake is determined using an electronic torque gauge. A plot of torque against the 3/4 power of time gives a linear fit to the data. The slope of this line is indicative of the sticking potential of the mud.
Laboratory and field data indicate that the test gives a good indication of mud condition and allows the user to investigate various options for reducing the sticking potential of the fluid. This information is useful both in the initial selection of mud type and in making technical and economic analyses of various mud treatment options. In addition to the laboratory evaluation described above, the sticking device has been used in several well-site tests with encouraging results. Some of these are described.
Stuck pipe is a major nonproductive cost to the drilling industry. A survey presented by a major drilling contractor in 19921 estimated that over a 15 month period, 36% of all reported drilling problems worldwide resulted from stuck pipe.
Generally, stuck pipe problems are divided into two categories: mechanical sticking (up to eight subsets have been identified2) and differential sticking. The proportion of incidents classified in each category varies with the type of well and the geographical area. For example, one oil company estimated that in its North Sea wells, 29% of the cost associated with stuck pipe was caused by differential sticking and 70% from mechanical sticking. Conversely, in the Gulf of Mexico, differential sticking was the dominant cause for 61% of the total cost of nonproductive incidents.3 The same paper estimated that the cost of stuck pipe to the industry is in excess of $250 million each year.
In this paper, the authors focus on the problem of differential sticking. The mechanisms of differential sticking are examined, along with a brief review of the published experimental methods that have been used to investigate the phenomenon, particularly those which relate to mud formulation and properties. Furthermore, a simple differential sticking test (called for convenience the "Stickance" test) will be described, along with its application in the laboratory and at the well site for the study and prevention of differential sticking.
Mechanisms of Differential Sticking
Helmick and Longley4 and Outmans5 first proposed the mechanisms of differential sticking in the late 1950's.
Differential sticking occurs when a part of the drillstring, casing, or logging tool becomes embedded in the mud filter cake and is held firm by mud pressure that exceeds the formation pressure. Differential sticking can only take place across permeable rock formations, such as sandstones, where a mud filter cake builds up during drilling. It does not occur in shales and other very low permeability formations where mud filter cakes normally do not form. Generally, differential sticking also only occurs when the drill string or tool is stationary (or sometimes when it is moving very slowly) across the permeable zone. Typically, the occurrence of differential sticking can be diagnosed when the drillpipe cannot be rotated or moved up or down, but unrestricted mud circulation is still possible. This is in contrast to a form of mechanical sticking.
Significant mud overbalance, as well as an exposed permeable section, must also exist for differential sticking to occur. The exact value of overbalance above which the risk of sticking becomes high is not clear, though it may well be dependent on formation characteristics, hole angle, hole size, bottomhole assembly (BHA), and drillpipe contact area with the permeable formation, mud type, and mud properties. Several "critical" values have been proposed over the years, ranging from a few hundred to the low thousands of pounds per square inch. However, what is clear is that as many reservoirs become depleted, a significant number of wells will be drilled with high overbalance pressures, thereby maintaining the industry's concerns over differential sticking.
The likelihood of differential sticking is increased further with the length of the permeable section that is open to the drilling fluid. The continued trend towards extended reach and horizontal drilling means that increasing lengths of permeable formations are exposed. Consequently, the prevention of differential sticking will remain a high priority in development drilling. Clearly, the nature of the rock formations encountered certainly cannot be altered. Therefore, if those formations carry an intrinsically high risk of differential sticking, this has to be accepted. Also, high overbalance pressures may be unavoidable if they are needed to maintain well control or wellbore stability in other parts of the openhole section. However, mud composition and properties can be modified, within limits, making fluid formulation and engineering a prime and flexible weapon in the prevention of differential sticking. The need to optimize mud properties becomes apparent when one considers the mud variables that are known, or are assumed, to influence differential sticking. These are:
- mud overbalance (i.e., the mud density, although as stated above, this may be constrained for operational reasons);
- mud solids content (both high-gravity and low-gravity solids); while the amount of high-gravity solids is generally fixed, or at least confined within quite narrow limits, considerable control over the drilled solids content can be exercised;
- generic mud type;
- specific mud formulation, including the presence of additives such as lubricants and bridging/cake-forming particles;
- fluid loss; and
- filter-cake quality:
- cake thickness,
- cake lubricity, and
- cake strength.
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