MODIFIED MUD REDUCES BOREHOLE ENLARGEMENT IN EGYPT

Osama el Ibiary, Ahmed Shimi, Mohamed el Zayat, Hisham Ramadan
Wepco
Alexandria, Egypt

John E. Hostettler
Drilling Specialties Co.
Bartlesville, Okla.

David H. Beardmore
Phillips Petroleum Co.
Bartlesville, Okla.

A redesigned mud system reduced borehole enlargement in shale to an average 2 in. from 8 in. in an Egyptian Western Desert well.

At least one of the three main changes that were made in the mud system was responsible for the improvement.

The three modifications were:

Change the mud from brackish to 15% NaCl
Increase the concentration of shale-stabilizing sulfonated asphalt to 6 lb/bbl
Raise the mud weight by 0.5 lb/bbl.

The redesigned mud was relatively easy to run, but more improvement is needed because the worst of the remaining borehole enlargement is occurring near potentially productive zones. Also, ways to reduce the cost of the mud without affecting performance should be studied.

CONTRACT AREA

Phillips Petroleum Co. and its coventurer are exploring the South Umbarka block in the Western Desert of Egypt (Figs. 1 and 2).

Western Desert Operating Co. (Wepco) is conducting the drilling operation for Phillips and its coventurer.

Wepco is a joint venture of Phillips and Egyptian General Petroleum Co. (EGPC).

The Theoris Ig 17-1, drilled in late 1988, was the first well of a renewed exploration program in the South Umbarka block conducted by the Phillips group. Fig. 3 shows gamma ray and caliper logs of the well, which indicate that there was severe borehole enlargement in the shale sections below 9,500 ft.

Among the problems that occurred were tight hole, stuck logging tools, and log interpretation difficulties.

Khepri Ig 14-1, the second exploratory well, was drilled in 1989.

That well also encountered drilling problems associated with borehole enlargement in the shale sections (Fig. 3).

These problems included stuck pipe (resulting in a sidetrack), tight hole, caving shale, a lost logging tool (this made it impossible to test one potential hydrocarbon zone), stuck logging tools, and log interpretation difficulties.

Based on the extent of these problems, a special effort was made to improve the borehole condition in the next well, the Sethos Ig 14-2. Because most of the borehole enlargement occurred in the shale sections, the new mud program was focused on stabilizing the shale.

The improvements to the mud program were based on a lithological analysis of cuttings from the Theoris Ig 17-1 well, lab reactivity tests on cuttings from the same well, and application of generally known shale stabilization principles.

LITHOLOGICAL ANALYSIS

A lithological analysis of the cuttings from the Theoris Ig 17-1 revealed that:

The volume of shale cuttings was greater than expected for the amount of shale drilled as determined from the wire line logs. This indicates that a substantial fraction of the cuttings were actually carvings from uphole shale sections.
The types of shale cuttings were relatively consistent over the interval studied, 6,500-12,400 ft.
Most of the cuttings were elongated and splintery, and their shape indicated significant weakness along bedding planes.
The reddish iron staining of many samples may indicate that some shale zones were of a fractured nature.
The shale cuttings had some tendency to disintegrate in water.

The two most important points determined by this analysis are that the shales tend to be physically weak due to fractures or separation along bedding planes, and that they have some tendency to react to fresh water.

LAB TESTS

Cuttings from the Theoris Ig 17-1 well were exposed to various clear fluids to determine their reaction to various salts in solution.

Cuttings from four different depth intervals between 6,500 and 11,600 ft were used. The shale cuttings (or carvings) from each interval were classified into one of six shale-type categories, depending on the shape and color of the shale pieces. Most of the cuttings were thin, flat, "splintery" pieces that had broken along bedding planes.

These shale samples were then exposed to various clear fluids, including saturated NaCl, 3% KCI, distilled water, 15% NaCl, and 1.2% NaCl. (This last fluid simulated the salt content in the muds used at the previous two wells.)

All of the salt solutions contained 1 ppg of a polyanionic cellulose polymer (PAC). The condition of the cuttings after being placed in these solutions, which was qualitatively evaluated by a visual inspection, was recorded.

Even though some of the shale types were swelling shales, most of the shale types did not swell and disintegrate, but split into smaller slivers on exposure to the fluids. The saturated NaCi/PAC solution was best at preventing disintegration and splitting, followed closely by the 15% NaCl/PAC solution.

The 3% KCI/PAC solution was somewhat less effective, while the distilled water caused the most disintegration. The 1.2% NaCl/PAC solution was almost as bad as the distilled water, indicating that the fresher water used in the previous two wells may have contributed to shale destabilization.

Table 1 lists the performance of the various fluids in the fluid exposure tests,

Based on the these tests, it was decided that the water phase of the revised mud system should contain 15% NaCl.

IMPROVED MUD

The mud system used in Theoris Ig 17-1 and Khepri Ig 14-1 was a brackish water (8,000 ppm chlorides)/PAC/sulfonated asphalt mud (Table 2).

The revised mud program for the Sethos 14-2 well, drilling in late 1989, was designed to employ four basic principles of shale stabilization:

Use of salt to inhibit the swelling of hydratable clays. Based on the lab tests, 15% NaCl would inhibit swelling of the clays in the shale.
Use of an asphaltic shale stabilizer to minimize penetration of mud filtrate into the fractures and bedding planes in the shale that the lithological analysis revealed. The concentration of asphalt additive (sulfonated asphalt-Soltex) was increased to 6 ppg in the improved mud.
Increase in mud weight to stabilize the mechanically weak, fractured shale. Addition of 15% NaCl increased the mud weight from 9.0 to 9.5 ppg. It should be noted that this mud weight increase might cause a slight decrease in rate of penetration.
Maintaining a laminar fluid flow regime in the annulus to prevent erosion of the weak, fractured shale. This was already a part of the original mud program, and was also closely monitored in the Sethos Ig 14-2 well.

The composition and properties of the revised mud system are listed in Table 2.

The 15% NaCl/PAC/sulfonated asphalt mud was used in the third exploratory well in the South Umbarka block, the Sethos Ig 14-2 well.

BREAKOVER

Due to the fact that a formation at about 7,000 ft required a fresh water mud during an intermediate logging run, the brackish water mud used in previous wells was also used in the Sethos Ig 142 well until a depth of about 8,000 ft was reached. At about 8,000 ft, breakover to the 15% NaCl salt mud was made.

The following steps were taken to minimize flocculation during breakover:

A pilot test was run to determine the effects of the breakover on mu d properties.
All solids control equipment was run during trips, coring, and circulation, as well as during drilling to minimize solids in the mud prior to breakover.
The mud was diluted with a water/PAC blend to reduce solids to 4% by volume and to reduce the bentonite content to 10 lb/bbl.
15% NaCl was added to the system.
Further additions of bentonite required prehydration in fresh water and treatment with thinner to provide an adequate yield in the salt water. Sulfonated asphalt was also prehydrated in fresh water to maximize solubilization before being added to the salt mud. Table 3 summarizes the mud properties before and after the actual breakover. No significant problems were encountered during the breakover.

SYSTEM MAINTENANCE

It was very important to maintain the salt concentration by checking the chlorides concentration several times a day and by adding any needed salt.

Maintenance of PAC and sulfonated asphalt was required as drilling progressed because these materials coated drilled solids and, therefore, were removed from the system as the solids control equipment removed the cuttings.

WELL COMPARISON

The purpose of the revised mud system was to minimize the tight hole, caving, and log interpretation problems that had occurred during the drilling of the Theoris Ig 17-1 and Khepri Ig 14-1 wells.

After breakover to the new mud system, about 3 hr of lost time were incurred during drilling due to tight hole, circulating carvings, or washing and reaming tight spots. The other two wells averaged about 5 hr of such lost time.

No time was lost on the Sethos Ig 14-2 due to stuck logging tools. The log analysts indicated that the quality of the log response was significantly improved due to the reduction in borehole enlargement.

CALIPER LOGS

Caliper logs, which give hole diameter, were analyzed to determine the effectiveness of the 15% NaCl/PAC/sulfonated asphalt mud system.

The "delta caliper" log is the difference between bit gauge and the caliper log. The log was used as an indication of borehole enlargement due to shale caving.

Delta caliper logs of the Khepri Ig 14-1 and the Sethos Ig 14-2 (Fig. 3) were compared in detail. The Theoris Ig 17-1 was omitted from the detailed comparison because of its significant distance away from the two wells. However, it was clear that the borehole enlargement due to shale caving was worse in the Theoris well than in the Sethos well.

The evaluation method was based on measuring the area under the delta caliper curve on the computer processed log.

This area is an indication of the severity of the borehole enlargement and was tabulated for each of the two wells on a formation-by-formation basis.

For example, the enlargement (area under the delta caliper log) in the Khepri Alam el Bueib A formation was 1,152 units, and the enlargement in the Sethos Alam el Bueib A formation was 336 units. To correct for the fact that the shale zones are of different thicknesses in the two wells, the average amount of borehole enlargement in shale was calculated by dividing the area under the delta caliper curve by the thickness of the shale zones in each formation. The thickness of the shale zones was determined by the lithology log.

This calculation results in an average borehole enlargement in shale for the particular formation being analyzed. For example, the average borehole enlargement in shale in the Khepri well Alam el Bueib A formation was 5.5 in. compared to 1.5 in. for the Sethos well (Table 4).

After all of the formations from the Alamein dolomite (about 10,000 ft) down through the Alam el Bueib D5 sand (about 13,300 ft) are considered, the average borehole enlargement in shale in the Khepri well was 8.2 in. and that in the Sethos well was 2.3 in. This represents a 72% reduction in borehole enlargement in shale.

The mud cost for the Sethos well was 1.4-1.5 times that of the previous wells. It was considered a worthwhile expenditure to avoid the drilling, logging, and log interpretation problems that had been previously experienced.

Analysis of the caliper logs from the Sethos well also indicates that there still exist shale zones that suffer from borehole enlargement of as much as 5 in. Therefore, further improvements need to be made to this mud system.

Possible additional improvements include a further increase in mud weight, and incorporating an additional fluidloss material designed to prevent penetration of mud filtrate into the fractures and bedding plane partings in the shale. It remains to be seen if the additional mud expense will be offset by the benefits of a nearer-to-gauge well bore.

ACKNOWLEDGMENTS

We would like to thank Wepco and Phillips Petroleum Co. for permission to publish this article.

We also would like to express our appreciation to W.O. Chapman, G.B. Heckler, G.G. Dixon, R.G. Littlefield, and W.T. Siemers for the their valuable input to this project.