A successful kill fluid formulation for the overpressured Jilh formation has been achieved. Jilh formation is a dolomitic middle Triassic tight zone that ranges from 8,000 to 10,000 ft TVD with abnormal salt-water pressure that requires a kill fluid with a density up to 150 lb/ft 3 (pcf). Addition of weighting materials such as barite (BaSO 4 ) or hematite (Fe 2 O 3 ) is needed to achieve the desired density for the kill fluid. However, maintaining high volume of solid content for barite particles (or any other weighting material) in suspension is challenging, especially for extended period of time. This phenomenon is described in the industry as static barite sag and it may lead to serious well control incidents and lost circulation.Saudi Aramco developed a new formula for kill fluids at high range of densities up to 150 pcf using barite with manganese tetroxide (Mn 3 O 4 ). No similar formulations were developed before to the best of the authors' knowledge. The properties of small particle size (D 50 ϭ1 m), spherical shape, and high specific gravity (4.95 g/cm 3 ) of manganese tetroxide make it a good weighting material to reduce solids loading and settling compared to barite (SGϭ 4.20 g/cm 3 and D 50 ϭ20 m). Results show that the density variation between the upper and lower section after 24 hours of vertical static condition is 5 pcf for the new formula (densityϭ 150 pcf) compared to a variation of 13 pcf for the conventional formula that has barite only. When the drilling fluid is put in inclined static condition for 24 hours, the density variation is 6 pcf for the new formula (densityϭ 150 pcf). However, the conventional formula showed a density variation of 17 pcf.The experimental work in this paper involved rheological properties, thermal stability, HT/HP filtration, and static sagging. This paper also describes field cases with barite sagging in high temperature wells and method and formulation of combining manganese tetroxide and barite to formulate new kill fluid formulations for Jilh Formation in Saudi Arabia fields.
Horizontal wells drilled in the Marrat (lower Jurassic), Minjur and Jilh (upper and middle Triassic) formation in Saudi Arabia are known to present severe stuck-pipe challenges. Stuck pipe in a horizontal well is a problem that can result in loss of tools, non-productive time, sidetracks and loss of well thereby increasing the wellbore construction cost. Thus to mitigate the problems arising due to stuck-pipe issues, a new drilling fluid was formulated using a combination of barite and manganese tetroxide (Mn3O4) as weighting agents. The use of such a combination as a weighting material in a drilling fluid provides both operational and monetary benefits. Mn3O4 with a smaller particle size (D50=1 µm), spherical shape, and high specific gravity (4.95 g/cm3) makes it a good weighting material to reduce solids loading and settling compared to barite (SG= 4.20 and D50=20 µm). The fact that manganese tetroxide is also acid-soluble provides more operational benefits when a fluid with a combination of barite and Mn3O4 is used as weighting agent. In the event of a stuck-pipe incident during drilling, the use of acids or acid precursors would result in the dissolution of the Mn3O4 thereby resulting in a partial breakage of the manganese tetroxide-barite filter cake formed in a well during drilling. This partial breakage of the filter cake is expected to free the pipe thereby mitigating the problems associated with stuck pipe incidents. The use of a barite along with Mn3O4 would also result in a reduced fluid cost as compared to a fluid formulated with Mn3O4 alone as the weighting agent. Thus, this paper showcases the benefits of using such a combination of barite and manganese tetroxide as weighting materials in 100pcf, 120pcf and 150pcf drilling fluids hot rolled at 250°F/300°F. The experimental work in this paper involves measuring rheological properties, thermal stability, HTHP filtration and static sagging resistance of the fluid at 250°F/300°F. The paper also describes the results of using different acid based filter-cake breaker fluids for the partial dissolution of manganese tetroxide based filter cake. A blend of manganese tetroxide and barite provides increased sag resistance as compared to a drilling fluid formulated with barite alone as the weighting agent.
Invert-emulsion fluid (IEF) systems formulated without organophilic clay have successfully addressed issues of barite sag and reservoir productivity. While ‘clay-free’ IEFs provide robust properties, additional materials in the form of low-gravity solids (LGS), like sized calcium carbonate or clay-type materials are required to bolster the rheological properties and suspension character of the system. This increases the plastic viscosity of the fluid, thereby resulting in a lower rate of penetration (ROP) and higher equivalent circulating densities (ECD). In the absence of LGS, the loss of suspension, especially in an inclined well, can lead to sag and cause a density gradient along the fluid column that may fracture the formation. This paper presents a solution towards sag in an IEF that is free of LGS viz. fine sized calcium carbonate or clay-type material, and is formulated with a novel sag control additive to suspend barite. The additive was shown to prevent sag in 9-ppg, 12-ppg, and 16-ppg clay-free IEFs in a static-aging cell held vertical at 90° and then inclined at 45°. The additive provided sag control in different base oils with less than 5-mL oil separation. Extended sag testing with the sag-control additive at 250°F for 24 hours and at 150°F for 60 hours gave minimal oil separation and no barite sag. High temperature-high pressure (HTHP) testing on 9-ppg and 12-ppg demonstrates a flat rheological profile in the presence of this additive. Contamination testing of IEFs containing this additive shows that the suspension characteristics remain unaffected and the mud properties are tolerant to the presence of contaminants. The additive is North Sea compliant with a biodegradability of 71.4% in 42 days and LC50 of >10 g/mL for aquatic organisms. Experimental data demonstrating both the environmental and anti-sag performance of the novel sag control agent is presented and compared to fluids without the sag control agent.
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