Silicate Based Muds: Chemical Optimization Based on Field Experience
- I. Ward (BW Group plc) | J.W. Chapman (BW Group plc) | R. Williamson (Mobil North Sea Ltd.)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- March 1999
- Document Type
- Journal Paper
- 57 - 63
- 1999. Society of Petroleum Engineers
- 2.4.3 Sand/Solids Control, 1.6 Drilling Operations, 4.2.3 Materials and Corrosion, 2.2.3 Fluid Loss Control, 1.14 Casing and Cementing, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 5.2.2 Fluid Modeling, Equations of State, 4.2 Pipelines, Flowlines and Risers, 1.8 Formation Damage, 1.11 Drilling Fluids and Materials
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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 trials 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.
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muds, were successful at drilling heaving shales1 but the control of their rheology proved difficult and they were superseded by the introduction of natural organic dispersants to treat bentonite muds.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 trials were undertaken in the 1960s by Darley which again failed to establish silicate based muds as accepted systems.2 In August 1994 a new generation of silicate muds was run in the Southern Sector of the UK North Sea. The background and working mechanisms of silicates have previously been documented.3,4 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.
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 was first examined and it was seen that the viscosity of silicate solutions is at a minimum at molar ratios between 1.8:1 and 2:1. At ratios higher than 2 and lower than 1.8 the viscosity increases rapidly. Each of these effects was dramatically increased as the solids content of the silicate solution increased from 30% to 45%.
Shale recovery tests [American Petroleum Institute (API) recommended Practice 13I, 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. Figs. 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 NaCl 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 was 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 polymerize 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 dilute the molecular ratio.
The higher pH has the effect of reversing the polymerization of silicates. This respeciation is a slow process as complex colloidal structures return via cyclic species to smaller chains with a small proportion of monosilicate. At high concentrations the rate of polymerization is higher and the rate of respeciation lower. The effect of this process on silicate solutions was illustrated by a precipitation test on a high ratio silicate with 60 ppb KCl added. Precipitation was over 50% but with the addition of 1 ppb of caustic soda this was reduced to below 2%. This therefore was seen as the primary method of controlling polymerization and the resultant precipitation/gelation.
Additional testing was carried out to assess the silicate mud's stability to contamination by seawater, clay, calcium chloride, cement and Zechstein brines together with elastomer compatibility testing. It was seen that the fluid was largely unaffected by seawater and produced results equivalent to a standard polymer fluid.
Contamination with divalent ions produced increases in initial rheology which reduced after hot rolling. It was established that precipitation of the silicate due to the presence of divalent ions would not have an adverse effect on the rheology of the fluids. Contamination with Oil Companies Material Association (OCMA) clay gave initial increases in rheology which thinned back to normal levels after hot rolling.
Elastomer compatibility studies showed that rubbers exposed to silicate fluids produced a small degree of hardening which was well within acceptable limits. Silicates are used as fillers in the manufacturing of various rubbers.
The inhibition provided by the silicate muds had initially been assessed by using the slake durability test.5 It was evident that in formulated fluids this test gave very good recoveries with many types of inhibited fluid. To try to differentiate between these fluids, a new test was developed to measure the hardness of the shale pellet after exposure. A penetrometer was used to measure the load required to penetrate a standard depth into the pellet after it had been exposed to various fluids. Fig. 4 illustrates the relative inhibition in terms of softening for a range of fluids and shows the superior performance of silicate based muds over glycol, and other water based systems.
After this laboratory work two silicates were selected for field trial. One had a low molecular ratio (2:1) and one a high (3.3:1) molecular ratio; both had high solids content.
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