Application of Silicate-Based Drilling Fluid in Tertiary Clays Offshore Norway

Abstract

Application of Silicate-Based Drilling Fluid in Tertiary Clays Offshore Norway J.L. Barnfather, SPE, Baroid Drilling Fluids; D.J.M. Bax, SPE, Norske Shell E&P; E. van Oort, SPE, Shell E&P Technology Co.; O.J. Nielsen, SPE, Baroid Norway S.A. Abstract A water-based mud based on soluble silicates has been used to drill through the highly reactive Tertiary clays and marls of the Nordland/Hordaland group and the Ekofisk chalk on two sub-sea exploration wells offshore Norway. The silicate-based fluid was primarily selected for its superior clay and shale stabilizing characteristics and was regarded as an environmentally-friendly alternative to oil-based mud and synthetic-based mud, which are precluded from use by the operator's environmental protection policy. Additional factors were anticipated cost-effectiveness and an expected reduction in total chemicals discharged compared to other WBM's. The performance of the drilling fluid, which actually combines a soluble silicate and a polyglycol for added performance and mud stability, suggests that a new milestone has been reached in finding a cost-effective WBM with low environmental impact to replace OBM's, SBM's and low-efficiency WBM's currently available, in a wider range of drilling applications than had previously been supposed. This paper presents the learning opportunities during these field trials. Data for drilling efficiencies are reviewed which will demonstrate the fluids superior performance compared to other WBM technologies. A cost comparison to mud systems used on offset wells is made, and savings realized against well AFE are presented. Silicate based drilling fluids: General Introduction Silicate-based drilling fluids have been successfully reintroduced in the field in recent years after having been pioneered in the early 1930's and briefly revisited by Darley in the late 1960's. The current generation of silicate-based polymer mud systems contain typically 5–15% v/v of a soluble silicate product, for which the generic formula is given by M2O.xSiO2, where M is Na for sodium silicate and K for potassium silicate and × represents the molecular ratio. Soluble silicates arc generated by either fusing quartz (sand) with soda ash or potash, or by dissolving quartz in NaOH (caustic soda) or KOH (potassium hydroxide) at elevated temperatures. Silicates in stable solutions at high pH (typically 10.5–12.5) usually encompass a variety of different molecular species, ranging from monomers to low-molecular weight oligomers. The negatively charged oligomers will rapidly precipitate polyvalent ions such as Ca++ and Mg++, and may form three-dimensional gel structures when solution pH is lowered. Shale and chalk stabilization. Although the exact mechanisms by which shale and chalk destabilizes are quite complex (see ref. 4), the solution to stability problems appears to be to engineer the mud system in such a way that mud pressure will be confined to the wellbore (i.e. pressure is not penetrating the formation) and that water cannot reach the reactive chalk and clay surfaces. Silicates achieve this by reacting with the Ca++ and Mg++ ions present on chalk surfaces and in shale pore throats and by gelling in the low-pH pore fluids of shales. The precipitates/gels formed will plug and coat the borehole wall, thereby preventing water and mud pressure's from entering the formation. It has been demonstrated that the silicate precipitate/gel in shale pores also acts as an almost ideal semi-permeable membrane, allowing the osmotic flow of water. By using elevated levels of a monovalent salt (e.g. NaCl, KCl) in the silicate-based drilling fluid to suppress water activity, osmotic flow of pore fluid from the shale to the mud may be induced. The lowering of the near-wellbore shale water content and pore pressure can act as a secondary shale stabilizing mechanism, particularly useful in stabilizing weak, reactive clay/shale formations. The addition of K+ to silicate-based fluids in the form of KCl has the additional advantage of removing Ca++ and Mg++ ions from clay surfaces by ion exchange, which can then become available for more rapid precipitation of the silicate oligomers. P. 51^