TECHNOLOGY Monitoring pressure buildup helps determine hole cleaning efficiency

V.A. Hurshudov
Feniks Co.
Moscow
N.I. Andrianov
SevKavNIPIgas
Stavropol, Russia

A simple, reasonably accurate method of predicting hole cleaning efficiency during drilling only requires control of pump pressure and pulling the drill stem off bottom momentarily.

The ability to predict the degree of hole cleaning during drilling through an unstable formation can help prevent stuck pipe.

Hole cleaning is one of the most important functions of a drilling mud. Cuttings can accumulate in a hole because of borehole enlargement, caving and sloughing, inadequate annular velocity, and certain flow characteristics of a mud. These factors can lead to problems such as slacking off and sticking of the drillstring, pressure buildup on pumps, and plugging of the annulus by cuttings.

Predicting the degree of hole cleaning can help an operator adjust drilling parameters to improve the efficiency of cuttings removal from the well bore before a problem occurs.

Various methods have been proposed for predicting hole cleaning, and in general they may be classified as quantitative or qualitative.1-3

Quantitative methods are based on evaluation of volumetric cuttings concentration in the annulus (cuttings concentration values 5% have been shown to cause problems) or cuttings transport ratios (hole cleaning efficiency).1-3 There are some important situations not handled by these methods because they were developed with the assumption of only single-sized cuttings being transported from the bottom of the well and do not include sloughing from the well bore wall. In drilling unstable formations, it is essential to consider problems from sloughing. Therefore, none of the methods proposed in the literature for quantitative prediction of the degree of hole cleaning are suitable for field application. They can be used as an investigative tool for postanalysis of hole cleaning problems, however.

Qualitative methods are based on monitoring mechanical and hydraulic parameters of the drilling process. In practice, it was found that sludging of the annulus is accompanied by pressure buildup on pumps, an increase in rotary torque, a decrease in penetration rate, and slacking off and sticking of the drillstring. These are also symptoms of packing, which requires a different set of technological measures to eliminate.

Pressure monitoring

The method of using pump pressure monitoring for hole cleaning analysis was based on a study of wells in which there was progressive plugging of the annulus by cuttings and sloughed material during drilling out unstable formations.

Patterns were found between hookload and pump pressure fluctuations when the bit was pulled off bottom.

As shown in Fig. 1 [1,700 bytes] (Curves 1 and 2), in a normal situation, pulling the bit off bottom is accompanied by a typical decrease in pump pressure. In a case of sludging, each time the bit is pulled off bottom there is a pressure buildup some time before the annulus plugs (Curves 3 and 4 in Fig. 1 [1,700 bytes]). Curves 5 and 6 in Fig. 1 [1,700 bytes] indicate the annulus has plugged, and the drillstring is stuck.

The following model was developed to explain this phenomenon. Sloughing from the well bore wall occurs continuously during drilling an unstable formation. When the cuttings carrying capacity of the mud is insufficient, the amount of rock particles in the annulus increases. During circulation, these particles are transported into zones of borehole enlargement. In these zones, the annular mud velocity decreases, and particle concentration increases. As a result, a batch fluidization state is attained. (the batch fluidization state is defined as the annular velocity at which the particles are distributed uniformly over the zone of borehole enlargement but are not taken away from the system.)

Abrupt fluctuations in the annular velocity occur when the drillstring is manipulated. The decreasing annular velocity when the bit is pulled off bottom results in a decrease in particle relative velocity (the difference between annular velocity and slip velocity of a particle). In the fluidized batch, the particle relative velocity becomes negative (the particle moves downward).

Differential pressure caused by the static head of fluidized particles increases, and pump pressure buildup occurs (Curves 3 and 4 in Fig. 1 [1,700 bytes]). In some cases, when the bit is pulled the fluidized particles are compacted and plug the annulus. Normally, this moment occurs when there is an abrupt pressure buildup (Curves 5 and 6 in Fig. 1 [1,700 bytes]).

Operation

Field applications in Russia have established reasonable validity of this model. Pump pressure buildup when the bit is pulled off bottom is a reliable method for qualitative estimation of hole cleaning.

The operator should determine the frequency of application of this method, depending on the stability of the formations drilled. In some areas, pressure buildup checks should be quickly made every meter of bit penetration or every 10-15 min during circulation. There is no need to change any mechanical or hydraulic parameters to prepare for these tests. Picking up the drillstring for a pressure check can be done quickly and simply, without disturbing operations.

If the pump pressure builds ups when the bit is pulled off bottom, special efforts are needed to intensify removal of cuttings and sloughed material from the borehole.

References

1. Iyoho, A.W., Horeth, J.M., and Veenkant, R.L., "A Computer Model for Hole Cleaning Analysis," Journal of Petroleum Technology, September 1988, pp. 1183-92.

2. Walker, R.E., and Holman, W.E., "Computer program predicts drilling fluid performance," OGJ, Mar. 29, 1971, pp. 80-90.

3. Bizanty, M.S., and Robinson, S.W., "Transport ratio can show mud carrying capacity," OGJ, June 27, 1988, pp. 39-46.

The Authors

V.A. Hurshudov is deputy director of Feniks Co. in Moscow. He previously worked for SevKavNIPIneft as head of the research laboratory and as the deputy director for the past 5 years. His experience includes deep well drilling in the Northern Caucasus region. Hurshudov has a degree in drilling engineering.

N.I. Andrianov is a senior researcher at SevKavNIPIgas in Stavropol, Russia. He previously worked in research for SevKavNIPIneft. He has research experience in cuttings transport in caved well bores. Andrianov has a degree in drilling engineering.