Abstract

Since the reintroduction of silicate fluids into the North Sea in 1994 over 45 hole sections have been drilled using these fluids. The performance of the fluid during the early field trails is discussed together with the compositional changes made as the envelope of performance was extended. The current state of the art in sodium silicate and mixed silicate formulations is presented together with field data to illustrate drilling performance.

Introduction

Addition of sodium silicate to water based muds was first undertaken in the 1930s. These systems known as protective silicate muds were successful at drilling heaving shales but the control of their rheology proved difficult and they were superseded by the introduction of natural organic dispersants to treat bentonite muds. Further field trails were undertaken in the 1960s by Darley which again failed to establish silicate based muds as accepted systems. In August 1994 a new generation of silicate muds were run by BW Mud and Mobil NSL in the Southern Sector of the UK North Sea. The background and working mechanisms of silicates have previously been documented. This paper describes the selection of a suitable silicate for use in drilling muds, the first field trial, subsequent modifications to the formulation and the continuing development of complimentary additives.

Product Selection

The remarkable inhibitive properties of soluble silicates have been known for many years. Producing a rheologically stable field mud was achieved by reasoned selection rather than exhaustive screening of potential products.

The effect of molecular ratio (SiO2:Na2O ratio) and solids content were first examined and it was seen that as molecular ratio increased beyond 2:1 so the viscosity of the silicate increased, and similarly at ratios of less than 1.8:1 but with less acuteness.

Each of these effects was dramatically increased as the solids content of the silicate solution increased from 30% to 45%.

Shale recovery tests (API recommended Practice 131, Hot Rolling Shale Particle Disintegration Test) were conducted on selected products and the relationship between increased molar ratio and improved inhibition was established (Fig. 1).

The tests were undertaken with brine silicate solutions rather than on formulated muds.

The lower molecular ratio products appeared to give a proportional increase in inhibition with concentration whereas the higher ratio silicates required a minimum effective concentration with a rapid improvement in performance between 5 and 10 ppb additions.

Further work was undertaken to assess the requirement of an electrolyte in the fluid. Fig. 2 and 3 illustrate the effect of salt concentrations on shale recovery.

With both KCl and NaCl it is apparent that the inhibition of the system is increased significantly when relatively low concentrations of salt are added. Additions of KCl and NaCI at 10 ppb were sufficient to increase the shale recovery from 76.5% to over 90%. Further additions beyond this level continued to increase recovery to in excess of 100% in both cases. Figures higher than 100% are due to product adsorbing onto the pellets.

The stability of the silicate brine solutions themselves were examined. Older solutions were seen to be cloudy due to a slow precipitation process. The higher ratio, and therefore less alkaline, silicates were most prone to precipitation. High concentrations of potassium chloride salt added to higher ratio silicates caused precipitation of over 50% in some cases. At pH's of less than 10 all silicates will polymerise to form a silica gel and the high buffering capacity of lower ratio silicates delays this effect. Additions of Caustic Soda to silicate fluids effectively dilutes the molecular ratio.

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