Rheological Characterization of Shale – Mud Interactions

1 Department of Geology， University of Benin， Benin City， Nigeria

2 Department of Chemical Engineering， University of Benin， Benin City， Nigeria.

*Corresponding author.

Received 16 October 2011; accepted 18 December 2011

Abstract

In a bid to identify a best drilling fluid for a problematic oil field in the Niger Delta region， rheological tests were carried out on three mud samples; BW1， BW3 and BW4. The results affirm that the load bearing capacity of XP-07 formulated as BW3 and BW4 in this investigation is excellent and fall within the same range or even better than those of REF Mud with a more than 90% drilling success history in Niger Delta. The rheological changes of XP-07 with increase in temperature and “assimilated”microscopic shale particles are very negligible and smaller than those of REF mud. XP-07 has been strongly recommended for all drilling operations in the problematic field. It has been re-emphasised as part of our recommendations that new guidelines for the close monitoring of drilling fluids supplied by mud companies and those actually used in the field (during drilling) be put in place.

Key words: Shale – mud interactions; Rheological characterization; Niger delta

INTRODUCTION

Mud properties are generally affected by the amount of shale “assimilated” during drilling (Akpokoje， 1994; Emofurieta Odeh， 2007). Increase in operational temperatures and pressures with depth usually lead to changes in the rheological properties of any mud system(Emofurieta， 1999， 2001). However， the resistance to change depends on the inherent properties of the respective mud systems (Omole et.al， 1989). Satisfactory performance of a mud is sometimes aided through the use of viscosifiers a lot of which degenerate and become non-effective under higher down-hole temperatures， meanwhile operational costs are jacked up into unusually prohibitive levels (Darnley and George， 1988， Falode et al， 2008). For avoidance of this， it is professionally better to drill with muds (which are compatible with the shales and of good thermal resistance and stability (SPDC Report， 1999). The degree of influence on the properties of the mud by formation rocks which in this case are the shales is assessed by observed changes in rheological parameters such as apparent viscosity， plastic viscosity (PV)， filtration loss， gel strength yield point (YP)， load bearing capacity and density before and after interaction with shale under varying thermal conditions (R D， NNPC， 1990). In this investigation， all the properties listed above (except density) were measured. The mud systems evaluated here include BW1， BW3 and BW4. A reference mud (REF) was used as control and for comparative purposes.

PROCEDURE: 165 ml of the mud (previously sheared for 30 minutes) was poured into the sample cup to reach its scribe line (or liquid mark) and placed on the support plate. The support plate was then raised up until the rooter sleeve was completely immersed to its own drawn line and tightened into position with a lock and a screw.

The “apparent viscosity” of the mud as indicated by the dial reading with the sleeve rotating at 600， 300， 200， 100， 6 and 3 rpm were measured at 76oF， 120oF， 160oF， 180oF， and 200oF using a viscometer. The measurements were repeated for each of the mud systems after the addition of 10gm and 20 gm respectively of -200 mesh mildly ground Tuns shale. At the end of each run， the mud was decanted and the solid (i.e. shale) deposited at the base of the sample cup was washed with acetone， dried and weighed and expressed as a function of load bearing capacity at high temperatures. The plastic viscosity (PV) in centipoises was calculated as the 600rpm reading minus the 300rpm reading while the yield point (YP) in Ibf/100 ft2 equals the 300rpm reading minus the plastic viscosity. The boiling temperatures of the muds and mud + clay mixtures were also recorded. The mud weight and water salinity were measured prior to commencement of the rheological readings. The results are presented in Table 1 while the graphical representations are provided in Fig.1.

Table 1

Rheological Properties from Low to High Temperature of Mud-Shale Solution

1. DISCUSSION OF RESULTS

The results are presented in Tables 1a – 1b and Figs. 1a– 1d. Generally， all the mud systems show very similar rheological characteristics. For example， the apparent viscosity， plastic viscosity， yield point and to some extent the gel of all the mud systems decrease with increasing temperature being slightly more so in the Ref. Mud than BW3 and BW4 as depicted by the higher gradient of the REF curve in the Apparent viscosity versus Temperature plots presented in Figure 1a. This clearly suggests that BW3 and BW4 are thermally more stable than the Ref. Mud. The percentage decrease in the 600 rpm of BW3， BW4 and Ref. Mud between the temperature range of 80oF and 220oF are 58%， 47% and 64% respectively while the corresponding decrease in 300 rpm are 54%， 40% and 62% respectively(Table 1a and Figure 1a). All the mud systems including the REF boil at between 200oF and 220oF although these boiling temperatures are expected to increase under downhole pressure conditions (Weber， 1975).

Figure 1a

Figure 1b

Figure 1c

Figure 1d

Figure 1

1a. Plot of Yield Point Against Temperature; 1b. Plot of Viscosity Against Temperature; 1c. Plot of Viscosity， Yield Point and Gel Strength Against Temperature; 1d. Plot of Viscosity， Yield Point and Gel Against Temperature

Table 2

Rheological Properties from Medium to High Temperature of Mud-Shale Solution

Shale Effect: In Figs.1b – 1d and Table 1b- 1d， the effect of adding 10gm and 20gm of different shale powder(-200 mesh) to the mud clearly indicate differing degrees of increases in the apparent viscosity of the mud systems. At room temperatures (i.e. 80oF)， BW3 shows an increase of 10 rpm (i.e.9%) for 10 gm OG and 39 rpm (i.e. 35%) for 20gm OG. 10gm OP recorded 23 rpm (21%) increase while 20gm OP increased the apparent viscosity of BW3 by 15 rpm (13%). On the other hand， the apparent viscosity of BW4 increased by 17 (15%) with the addition of 10gm OG. 20gm OG did not have any significant effect. With 10gm OP， there was an increase of 19 rpm (17%) and 53 rpm (47%) with 20gm OP. The apparent viscosity of the REF mud changed by 17 rpm (15%) with 10gm OG， 31 rpm (28%) with 20gm OG， 27 rpm (25%) with 10gm OP and 47 rpm (43%) with 20gm OP. At 180oF， BW3 increases in apparent viscosity are 7 rpm which is 12% with the addition of 10gm OP and 21 rpm (37%) with 20gm OP. BW4 increases by 6 rpm (11%) with 10gm OG， 25 rpm (45%) with 20gm OP. The REF mud increased by 5 rpm (10%) with 10gm OG， 13 rpm or 27% with 20gm OG， 9 rpm (19%) with 10gm OP and 12 rpm(25%) with 20gm OP. The shales did not affect the boiling temperatures of the muds (Weber and Daukoru， 1975).

All the plastic viscosities fall within the same range and also show systematic decreases with increase in temperature. However， the yield point (YP) of REF mud are significantly lower than those of BW3 and BW4. Consequently， the YP/PV of the REF are generally lower than one. API requires that the YP/PV of the mud be one or greater than one normally. This disparity is explicitly demonstrated in Fig 1d and Tables 1a – 1b. The gel strengths of BW3 and BW4 are also advantageously higher and fall within the expected range. The above analyses obviously prove BW3 and BW4 as better muds than the REF. That is not to say that REF is not a good mud， rather， that BW3 and BW4 are better favoured by all rheological considerations (Maron， 1969).

Gel Strength: Gel Strength is the direct measurement of the load bearing capacity or the ability of the mud to hold cuttings in suspension during connections or trips as well as continuously suspend weight material in the well. Gel strengths also have direct bearing on the swab and surge pressures created while pulling out of or going down the whole with the pipe. It is a determinant of the initial pump pressure required to break circulation (Murat， 1970).

The initial 5-， 10-， 15-， 30- and 60- minute gel strengths as well as the corresponding 30- minute values were all measured. The results in respect of BW1， BW3， BW4 and REF samples are presented in Table 2. The gel values are also presented in Table 1. Gel strength values of BW3 and those of the REF samples are identical. BW4 values are 10-35% higher than those of BW3 and REF. BW1 was below detection limit in all cases. This general trend is in conformity with the rheological parameters of both the pure muds as well as the mud plus shale mixtures. The above comparative analysis clearly identifies BW4 as the best formulation.BW3 and the REF samples are also good and would perform creditably well except to re-emphasis that BW4 belongs to a higher class with better rheology and thixotropy. BW1 is comparatively similar to an unweighted KCL water-base mud both of which are probably of lower grade.

Load Bearing Capacity of the Mud Systems: The relative load bearing capacities of the mud systems under investigation have already been insinuated from earlier discussions above. However， a direct estimation or measurement of the proportion of 12gm of fresh shale cuttings that can be held in suspension by the various mud samples was carried out in a dynamic state at room temperature and 200oF. The results are presented in Fig.1b. BW3， BW4 and the REF Mud held 100% of the ditch cuttings in suspension. BW1 (RG2) dropped nearly everything while another REF sample (RG1) held 61% of the ditch cuttings at room temperature but dropped everything at 200oF.

These results tally with the respective plastic viscosities， gel strengths and the thixotropic properties of the muds in general both under ambient and down –hole conditions. The load bearing capacities of the mud are expected to be enhanced by more than 80% under the influence of the mud pumping machine. An excellent oil well drilling mud must of necessity possess relatively high load bearing capacity to enable it evacuate ditch cuttings from the well during drilling(Emofrurieta Odeh， 2010a).Failure to do so invariably results in bottom piling/ sedimentation which can lead to stuck pipe and financial losses(Emofurieta Odeh， 2010b). To that extent， BW3 and BW4 are adjudged very suitable mud systems and strongly recommended for drilling operations in the troublesome oil fields.

CONCLUSION AND RECOMMENDATION

The results of the detailed investigation of the mud and mud + shale interaction of XP-07 has revealed that the mud BW3 and BW4 have very suitable rheological properties both under ambient and high Temperature/Pressure conditions. They are thermally more stable and less responsive to the effect of assimilated shale during drilling. Their load bearing capacities are comparable with that of the REF mud with more than 95% drilling success in Niger Delta.

Thus， XP-07 (BW3 and BW4) is hereby strongly recommended for use by the Oil producing company in Niger Delta drilling operations without the slightest reservation provided good drilling habits are maintained and necessary precautionary measures are taken.

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