Drilling fluids
This invention relates to drilling fluids, to drilling lubricants for such drilling fluids and to methods of drilling wells, particularly oil and/or gas wells using such drilling fluids. In particular, the invention relates to the inclusion of lubricants which afford lubrication in drilling fluids in which the lubricant compatibility and/or performance may be adversely affected by other components of the drilling fluid, particularly divalent cations such as calcium, formate salts or highly alkaline components such as alkali silicate.
1. A drilling fluid having an aqueous brine phase which includes at least one of:
a) dissolved calcium at a concentration of at least 0.1 molar,
b) dissolved formate at a concentration of at least 0.5 molar;
c) dissolved silicate at a concentration of at least 0.4 molar; or
d) brine salts at a concentration of at least 90% by weight of the concentration at saturation;
and further including at least one polyalkylene glycol and a lubricating agent which comprises at least one hydrocarbyl polyether phosphate.
2. A drilling fluid as claimed in claim 1 wherein the dissolved calcium is calcium chloride at a concentration of at least 0.5 molar.
3. A drilling fluid as claimed in claim 1 wherein the dissolved formate is an alkali metal formate at a concentration of at least 1 molar.
4. A drilling fluid as claimed in claim 1 wherein the dissolved silicate is an alkali metal at a concentration of from 0.4 to 1 molar.
5. A drilling fluid as claimed in claim 1 wherein at least one brine salt is present at a concentration of at least 95% by weight of the concentration at saturation.
6. A drilling fluid as claimed in claim 1 wherein the hydrocarbyl polyether phosphate is or includes a compound of the formula (I):
R 1 O.(AO) n .P(O)(OH) 2 (I)
where
R 1 is a C 6 to C 30 straight or branched chain alkyl, alkenyl or alkadienyl group;
AO is an alkyleneoxy group;
n is from 2to 100.
7. A drilling fluid as claimed in claim 6 wherein the group R 1 is a C 8 to C 22 , straight or branched chain alkyl, alkenyl or alkadienyl group.
8. A drilling fluid as claimed in claim 6 wherein all the alkyleneoxy groups, AO, are ethyleneoxy groups.
9. A drilling fluid as claimed in claim 6 wherein the alkyleneoxy groups, AO, are a mixture of ethyleneoxy and propyleneoxy groups having a molar ratio of ethyleneoxy groups to propyleneoxy groups of from 3:1 to 20:1.
10. A drilling fluid as claimed in claim 6 wherein n is at least 3 .
11. A drilling fluid as claimed in claim 1 wherein the hydrocarbyl polyether phosphate is present at least in part as an alkali metal or alkaline earth metal salt.
12. A drilling fluid as claimed in claim 1 wherein the concentration of the hydrocarbyl polyether phosphate in the drilling fluid is from 0.05 to 1% by weight.
13. A drilling fluid as claimed in claim 1 wherein the polyalkylene glycol is a polyethylene glycol having an average molecular weight of from 100 to 4000.
14. A drilling fluid as claimed in claim 1 wherein the 8% by weight.
15. A drilling fluid as claimed in claim 1 , further comprising at least one weighting agent, in an amount of from 1 to 75%, by weight of the drilling fluid.
16. A drilling fluid as claimed in claim 1 , further comprising at least one bridging agent in an amount of from 0.1 to 30%, by weight of the drilling fluid.
17. A method of drilling a well bore in which a drilling fluid is circulated through a drill string extending through a well bore, around a drill bit and returns in the space between the drill string and the wall of the well bore, in which the drilling fluid is as claimed in claim 1 .
18. A method of drilling a well bore down to a target, particularly a production, formation in which a drilling fluid is circulated down a drill string extending down a well bore, around a drill bit and upwardly in the space between the drill string and the wall of the well bore, in which the drilling fluid is as claimed in claim 15 .
19. A method of drilling-in a well bore in a production formation in which a fluid is circulated through a drill string extending along the well bore, around a drill bit and returns in the space between the drill string and the wall of the well bore, in which the fluid is as claimed in claim 16 .