Barney D. Love
BLC Systems
Houston
Michael N. Gilstrap
Avanti Services Inc.
Houston
Hugh Hay-Roe
Energy World Trade Inc.
Houston
A new procedure has helped eliminate severe lost circulation, even after one or more conventional lost circulation material (LCM) treatments failed.
This technology is based on precise, sequential pumping of size-ranked organic material (fibers and expandable particles) into the exposed pores of the thief zone.
No conventional treatment consistently solves lost circulation problems. The reasons for the inconsistency in treatment effectiveness are not well understood, but the following five factors are important:
- The characteristics of the drilling fluid can have a direct bearing on the frequency and severity of lost circulation.
A drilling mud formula that develops a strong filter cake is more likely to be effective in preventing lost circulation than a mud that develops a weak filter cake. The drilling mud cake helps seal minute cracks and narrow fracture tips.12
- Lost circulation is more likely to be a problem with oil-based drilling fluids than with water-based muds.3 4 Evidently the filter cake forms more effectively from water-based muds than from oil-based muds.4
With other factors equal, induced fractures propagate more readily and heal less readily if an oil-based mud system is used.
- The size and shape of LCM particles are critical. Thus, the LCM material must be carefully selected and prepared for a successful treatment .5 1 Many conventional materials may fail because the particles are the wrong shape, size, or texture.
- The sequence in which LCM is introduced into a problem well is critical. A common practice is to pump simultaneously a range of sizes of LCM particles. This method too often depends largely on luck, rather than on technology, for its success. A systematic approach controlling the size of material pumped may work better.
- Many treatments prove inadequate because pump pressures are kept substantially lower than the safe, optimum limit. The pressure at which a subsurface formation begins to fracture depends on several factors, including the tensile strength of the formation and the borehole condition, especially the width of existing cracks or fractures.2
Filter cake within a fracture, especially, a narrow fracture, limits the entry of fluid and thus helps prevent fracture propagation.4 A properly plugged-off formation is less susceptible to fracturing; hence, higher mud weights can be used safely.7
Nevertheless, typical lost circulation treatments do risk fracturing a susceptible zone. The larger, hard LCM particles can actually function as a proppant in some cases.
Statistics on lost circulation are difficult to obtain because most industry organizations (such as the American Petroleum Institute and International Association of Drilling Contractors) do not carry substantial data, and many operators do not release data from their experiences.
In a typical 10,000-ft well, the cost for failing to overcome lost circulation problems could range from as low as $5,000 onshore to millions of dollars offshore (possible loss of rig and well). It is understandable that operators do not publicize costly drilling problems. However, this lack of data creates difficulty in analyzing what materials and procedures worked well or not and why.
The consequence is a general lack of understanding of effective aids for lost circulation.
The unpredictable results of traditional treatments have lent an air of magic to the treatment of lost circulation. The customary trial-and-error approaches are not always effective and are often expensive. Lost circulation problems, however, can be solved logically and systematically.
ORGANIC FIBER LCM
The systematic approach uses a phased application of size-graded organic fibers and particles. This system, called Cell Block, uses particles ranging in diameter from around 0.12 in. down to microscopic size. The following characteristics were determined from several recent field trials in South Texas (Table 1):
- This system plugs-off thief zones sufficiently to allow weighting-up of the mud to handle high-pressure gas zones (as long as true fracture gradient is not reached). If the plugged-off zone begins to leak under the high mud weight, further treatment with the material can restore the effective seal.
- The organic LCM used in this system has been spotted safely with both water-based and oil-based drilling fluids at bottom hole temperatures up to 300 F. Several laboratory studies have indicated that few LCMs work well in oil-based muds, and those that work well are effective in controlling seepage loss only.3 The Cell Block technique, however, has worked well in oil-based muds (Table 1).
- In fibrous LCMs, the maximum size of the fibers and the gradation of sizes appears to be more important to fiber composition. This material uses limited-length fibers, eliminating the tendency to ball or string out. The Cell Block material has been used successfully with jets as small as 10/32 in.
- Tests in a filter press demonstrate that the material forms a thinner wall-cake than conventional LCM, thus reducing borehole bridging and swabbing effects.
- The fine organic particles produce a nonabrasive surface on the well bore wall, decreasing torque and drag by up to 40%.
The system has proven effective on three occasions this year in South Texas after numerous previous attempts to regain circulation were unsuccessful (Table 1). In each event shown in Table 1, earlier efforts to overcome the lost circulation (over periods ranging from 4 to 12 days) had failed. In less than 1 day, an application of the system worked in each case, even though some of the boreholes were difficult to work in because of the prior LCM treatments with widely differing materials.
This technology for effective sequential plugging can be designed to meet the varying geologic conditions of specific problem formations in most hydrocarbon-producing regions. An early version of the sequential plugging system was used successfully in drilling unconsolidated gravels and sands in Libya. In East Kalimantan, the same technique effectively maintained circulation in shallow, poorly consolidated gas sands.
The application of this LCM treatment requires sufficient data on thief-zone characteristics. The more information available, the greater the probability of success.
ACKNOWLEDGEMENT
The authors thank Fred Weigand and Denny Migl for their valuable technical information and assistance.
REFERENCES
- Nahm, J.J., Romero, R.N., Wyant, R.E., Hale, A.A., Briggs, B.R., Smith, T.R., Lombardi, M.A., and Keedy, C.R., "Universal Fluid: A Drilling Fluid to Reduce Lost Circulation and Improve Cementing," paper No. 27448, presented at the International Association of Drilling Contractors/Society of Petroleum Engineers Annual Drilling Technology, Conference, February 1994, Dallas.
- Morita, N., Black, A.D., and Fuh, C-F., "Theory of Lost Circulation Pressure," paper No. 20409, presented at the Society of Petroleum Engineers 65th Annual Technical Conference, 1990.
- Nayberg, T.M., "Laboratory Study of Lost Circulation Materials for Use in Both Oil-Based and Water-Based Drilling Muds," paper No. 14723, SPE Drilling Engineering, September 1987, pp. 229-236.
- Onyia, E.C., "An Analysis of Experimental Data on Lost Circulation Problems While Drilling With Oil-Base Mud," paper No. 22581, presented at the SPE 66th Annual Technical Conference, 1991.
- Loeppke, G.E., Glowka, A.G., and Wright, E.K., "Design and Evaluation of Lost-Circulation Material for Severe Environments," paper No. 18022, Journal of Petroleum Technology, March 1990, pp. 328-337.
- Nayberg, T.M., and Petty, B.R., "Laboratory Study of Lost Circulation Materials for Use in Oil-Base Drilling Muds," paper No. 14995, SPE Deep Drilling and Production Symposium, 1986.
- Fuh, G-F., Morita, N., Boyd, P.A., and McGoffin, S.J., "A New Approach to Preventing Lost CircuLation While Drilling," paper No. 24599, presented at the SPE 67th Annual Technical Conference, 1992.
