Deepwater drilling, fluid mechanics improved with pressure measurement

Dealing with cuttings loading, changing ECD

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PART III: This is the third of a three-part series on the various applications of downhole pressure monitoring while drilling. This article focuses on the day-to-day monitoring and interpretation of data, with the goal of improving drilling practices and monitoring mud properties.

The importance of acquiring real-time and recorded annular pressure measurements was demonstrated in the examples of shallow water flows, well control issues, and wellbore stability issues in the first two articles. In actual practice, Pressure-While-Drillingtrademark or PWDtrademark data are used for much more than detecting and monitoring major well problems.

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Comparing the annular pressure character of a water-based mud (WBM) with that of an oil or synthetic-based mud (SBM) reveals basic differences int the bottom hole EMW pattern over time.
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Monitoring and evaluating the less exciting and mundane data of day-to-day operations reveal much about mud properties, hole-cleaning issues, lost circulation issues, and drilling practices in general. Examples of each of these subjects are explored in this article. The drilling problems discussed in this month's article are common in deepwater drilling but are certainly not exclusive to it.

EMW responses

Analyzing and interpreting PWD data are sometimes complicated by the multitude of variables that can affect the character of the equivalent mud weight (EMW) curve on a PWD log. In reality, it is not possible to change one variable in a drilling operation and hold all others constant because both time and depth are two of those variables. Based upon experience, however, the effects of each variable can be established.

Typically, a water-based mud (WBM) system will achieve slower rates of penetration (ROP) than a synthetic-based mud (SBM) system. The higher ROP of a synthetic-based drilling fluid does not result in higher apparent cuttings load. Indeed, annular pressure curves indicate that there is less loading than with a water-based drilling fluid.

There are several plausible explanations for this effect. Utilizing a SBM results in a better gauge hole, whereas the WBM typically allows more washouts and ledges to form. The uneven cuttings load produced with a WBM is reflected in the uneven character of the EMW curve while drilling.

Sudden surges while drilling the stand and especially just before each connection using SBM are the result of restrictions or packing off in the annulus that is more common with WBM and results in a higher likelihood of becoming stuck.

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Changing mud weight and equivalent circulating density (ECD), due to variations in cuttings load and rheology, is common in drilling deepwater wells.
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Static pressures also differ for different drilling fluid systems. For normal drilling situations in deepwater, the static EMW in WBM is 0.1 - 0.2 ppg above the surface measured mud weight-in. Depending on depth, the static mud weight for a SBM is 0.2-0.4 ppg. The higher static EMW is attributed to the higher compressibility of SBMs, an effect magnified in the long cold risers of deepwater wells.

Cuttings load, mud rheology

Changing mud weight and equivalent circulating density (ECD) is common in drilling deepwater wells, due to variations in cuttings load and rheology. This assumes that all variables are constant except two - the actual cuttings load in the annulus and how it reacts with the mud. It is expected that the EMW increases slightly over time and depth, as annular frictional forces increase. Longer hole intervals result in a higher ECD. Static EMW is not affected by depth (assuming no compressibility) and should not change.

Situations where downhole mud density is constant and ECD is increasing, may be typical of a WBM with the cuttings load constant (hole cleaning is good). Mud rheology changes as the WBM reacts with the shale, increasing its viscosity. A situation with the mud density increasing and the ECD remaining constant, may be typical of a situation where the cuttings load is increasing (hole cleaning is poor).

Simply measuring the ECD (real-time PWD data can only be obtained while circulating) will show no difference between the two situations. How is the difference between increasing cuttings load and increasing ECD determined over time/depth intervals? Analysis of pumps off data can aid in answering this question.

The longer the pumps-off interval, the greater the increase in ECD. When the static EMW increases over time, other reasons need to be examined to determine the cause of this change. Increased cuttings load increases the density of the mud, resulting in a similar increase in static conditions. Cuttings load can change the viscosity of the mud as well, increasing mud frictional forces at a faster rate than depth alone.

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Significant increases of gel strength at drilling connections can result in excessive pump pressures to regain circulation, and prtentially exceed fracture pressures.
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Utilizing annular pressure measurements to determine leak-off test (LOT) pressures was discussed in the last article. These data can be used to determine an EMW upper limit during normal operations. If this limit is exceeded or ignored, well control and stability problems can become a problem.

Pressure problems

During drilling connections, it is apparent that mud develops gel strength quickly in the static conditions and requires significant pressure to break the mud gel at each connection. In some cases, this may be enough to initiate losses. However, not in one case. The mud weight was increased, and when the EMW reached the LOT pressure of 12.2 ppg, the formation broke down and lost circulation was observed at the surface. As in most cases, losses occur at the LOT pressure. It is equally likely that losses can initiate the LOT pressure because the well may have drilled into some weaker formations.

It is also common for a formation to break down at an instantaneous higher pressure surge, then sustain losses at a much lower value once the rock has been fractured. Often, a lost circulation incident is blamed on the formation or the mud. PWD data have shown that it can equally be drilling practices that are to blame (breaking gels, tripping too fast, reaming down, packing off; see Ward).

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Analysis of swab and surge pressures created by pipe movement reveals potential wellbore storage effects,or "breathing" in the well.
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One particular well was circulated to a lower EMW by stopping drilling and circulating the cuttings load from the mud. This took about 30 minutes, after which the well was circulated clean for a further hour without any reduction in EMW. This hour was essentially wasted time because the constant ECD showed either that the hole was clean, or, at that particular flow rate and rpm, no further cuttings were being disturbed (see Easton et al).

Tripping and reaming

Simple rig operations such as tripping and moving pipe can have a dramatic impact on the EMW downhole. In particular, a highly gelled drilling fluid can create significant swab and surge pressures even if pipe movement is minimal and is dependent on pipe running speed.

Trapped pressure in gelled mud can mimic the effect of fractures in the wellbore. In one well, the last pipe movement created a surge pressure, then some flow back occurred before the well became static. Conversely, the reverse is true if a swab is the last action and the fractures take fluid until reaching equilibrium.

Excessive swab and surges during pipe movement can also be caused by downhole, mechanical restrictions. Each time the rig up-reams or down-reams to alleviate a restriction downhole, a surge effect is observed at the PWD sensor that is higher than what it should be for an unrestricted annulus.

Up-reaming and down-reaming may create restrictions (packing off, balling) at stabilizers above the PWD sensor, and the drilling fluid does not easily circulate past them. This restriction may or may not be observed during normal drilling operations because the pipe speed is much less in magnitude than while reaming.

During normal operations, a formation pack-off can suddenly generate a "big" jump in EMW. Drilling immediately ceases and circulation is dropped to try and reduce the EMW levels. The EMW levels may not reduce much because the annulus is blocked, the upper limit effectively being the strength of the formation.

The pack-off and the float valve within the measurement-while-drilling (MWD) tool have trapped pressure at the bottom of the hole, and when the pumps are brought back online, the LOT can be exceeded and lost circulation the result. It is possible to move the pipe and free some of the pressure. This can induce excessive swabbing if not monitored properly. Usually, moving the pipe for several minutes frees the pipe of the pack-off.

This scenario places large stresses on the formation, potentially fracturing or collapsing it. Once a formation has experienced a situation like this, it never recovers properly and repeated problems can be seen in the same hole section. A typical scenario will have the pumps slowly brought back online and lost circulation material (LCM) pumped into the well. The driller then usually back-reams out of the tight hole until the pipe is relatively free and trips out normally.

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A typical formation pack-off, drilling scenario can generate excessively high EMW.
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Except for the initial pack-off, the EMW for the entire sequence is recorded data. A permeable zone open to the wellbore during these operations can create additional well control problems. Analyzing these data after trouble time can educate the driller on the consequences of each operation and the potential risks.

References

Ward, C., Andreassen, E., "Pressure While Drilling Data Improves Reservoir Drilling Performance," Paper SPE 37588, SPE/ IADC Drilling Conference, Amsterdam, March 1997.

Easton, M., Nichols, J., Riley, G., "Optimising Hole Cleaning by Application of a Pressure While Drilling Tool," Paper SPE 37612, SPE/IADC Drilling Conference, Amsterdam, March 1997.

Authors

Chris Ward is a Global Drilling Optimization Advisor with Sperry-Sun in Houston, specializing in MWD drilling tools and applications. He previously worked as a geologist for Arco in London before moving to the Norwegian operations of Sperry-Sun. He holds a BSc degree in geology and a PhD degree in geochemistry from the University of London.

Mitch Beique is a Drilling Engineer with Sperry-Sun, assigned to Halliburton's global deepwater solutions team. He has 21 years drilling engineering experience in North America, and seven years specializing in deepwater drilling. He holds degrees in petroleum and electrical engineering from Texas A&M.

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