Well Bore Fluid

The invention is illustrated with reference to the following Examples 1 to 7 and FIGS. 1 to 3 of the accompanying drawings.

FIG. 1 is a graph showing the relationship between the S.G. of a fluid and the concentration of a densifying agent in a solvent.

FIG. 2 and 3 are figures showing how the permeability of a core is affected by treatment according to the present invention.

In Examples 1-4, the test fluid according to the invention was that identified by the designation NODO 1, which is the name for a series of fluids of differing specific gravities prepared by dissolving differing quantities of a pentabromo diphenyl ether, sold under the Trade Mark DE-71 by Great Lakes Chemical Corporation, in a low toxicity drilling oil, sold by BP Chemicals Ltd under the Trade Name BP 8313, according to the relationship shown graphically in the accompanying FIG. 1.

The proportions can be chosen to give blends of specific gravity ranging from less than 1 to greater than 2.

Example 1 illustrates the low corrosivity of NODO 1.

The tests were carried out at 65-70.degree. C. for 165 hours on 4140 steel and carbon steel coupons in laboratory glassware using 350 ml of NODO 1 and 500 ml of an established completion fluid based on ZnBr.sub.2 /CaBr.sub.2 brine. NODO 1 has an SG of 1.91 and the brine of 2.31.

Results set out in the following Table 1 were obtained.

The above results indicate that the experimental completion fluid (NODO 1) is not as corrosive as the established completion fluid (ZnBr.sub.2 /CaBr.sub.2).

Example 2 illustrates the low level of attack on elastomers by NODO 1, which, in this example, has an SG of 1.7.

The performance of two elastomers exposed to NODO 1 and two comparative media were examined. Table 2 shows the initial physical properties of the elastomers used. NBR 689/4 was a conventional nitrile rubber (ex BP Chemicals Ltd) which has a high (41%) acrylonitrile content with a high (100 pph) loading of SRF carbon black and was vulcanized by a sulphur-donor cure system. Viton GF was a fluorocarbon elastomer obtained from James Walker and Co. Ltd., which was a peroxide cured terpolymer of vinylidene fluoride, hexafluoropropylene and tetrafluoroethylene. Both elastomers showed similar tensile strength data but differed in their modulii and consequent elongation. The nitrile rubber was compounded to possess good resistance to oil, whereas the Viton GF has more general chemical resistance.

The comparative media were a ZnBr.sub.2 /CaBr.sub.2 brine of SG 1.7 and a ZnBr.sub.2 / CaBr.sub.2 brine of SG 2.3.

The change in physical properties of the elastomers was measured after exposure to test fluid media for 28 days at 80.degree. C. and are reported as percentage property retention data in Table 3.

Comparison of the data of NODO 1 with the ZnBr.sub.2 /CaBr.sub.2 brine of SG 1.7 shows that in NODO 1 there was an improvement in the retention of mechanical properties of the nitrile rubber.

The ZnBr.sub.2 /CaBr.sub.2 brine of SG 1.7 was apparently less aggressive than a similar type brine of SG 2.3. Considerable stiffening of the nitrile rubber was evident in the 1.7 brine but this elastomer became so brittle in the 2.3 brine that it broke too early to allow a modulus measurement. The difference in behaviour between these brines is thought to be due to the fact that the 1.7 brine was of lower gravity than the 2.3 and hence the amount of zinc bromide (thought to be responsible for deleterious action towards nitrile rubber) in the former would be less than in the latter.

Example 3 illustrates the high thermal stability of NODO 1.

A sample of NODO 1 was held at 176.degree. C. (350.degree. F.) for 64 hours.

No change in SG or viscosity was noted thus indicating that the product was stable at elevated temperature.

The pentabromo diphenyl ether itself is relatively unstable, decomposing at temperatures between 220.degree. C. and 320.degree. C.

This example illustrates the non-damaging effect of NODO 1 on water sensitive structures and the damage done by a completion brine. Both fluids were of SG 1.8.

Two sample plugs were taken from a sandstone core containing about 10% by weight of swelling clays, mainly kaolinite (77-81% of the clay fraction), and smectite (18-23%).

The rock matrix was poorly cemented and the pore system was well developed.

Overall, from petrological data it was predicted that the rock might be liable to significant formation damage from aqueous fluids, as a result of swelling and dispersion of smectite and the mobilization of kaolinite particles. As the rock was inferred to be poorly consolidated, complete matrix disaggregation was envisaged to be a problem if the rock contacted incompatible water-based fluids. Thus, the chosen material was considered to be particularly sensitive to formation damage.

The core-fluid interaction tests were carried out at simulated reservoir conditions i.e. a confining pressure of 4800 psi, pore pressure 2741 psi and a temperature of 64.degree. C. The preserved plugs were flushed initially with kerosine to displace the crude oil. Their permabilities to kerosine were than established at steady-state conditions in forward and reverse flow directions. Oil based completion fluid or the conventional completion brine was then injected at a flow rate of 5ml min.sup.- 1 and at a pressure differential of 9.97 psi in.sup.- 1 for the oil based completion fluid, (14 pore volumes). The core permeability to kerosine was re-measured after the treatments, in forward and reverse flow directions.

(i) Water based completion brine

The first plug has an initial permeability to kerosine of 160 md, at a pressure differential across the sample of 2.7 psi in.sup.-1 . The pressure differential was kept low throughout the test to avoid mechanical damage to the rock and/or fines movement, as a result of high fluid seepage forces. After introducing 20 pore volumes of the conventional, water based ZnBr.sub.2 /CaBr.sub.2 completion brine (corresponding to fluid flux of 18.2 ml/cm.sup.3 of rock face) into the sample, the core's permeability to kerosine declined to 57.8 md in reverse flow, and 30.7 md in forward flow. The kerosine flow rate was maintained at 9.2 ml min.sup.-1 , with an average pressure differential of 11.52 psi in.sup.- 1across the core, after injection of the test fluid. This represented at 72% reduction in the plug's permeability as a result of the brine treatment. The reason for the discrepancy in the measurements with flow direction is not immediately apparent; these differences were not observed prior to injection of the test fluid. No fines were eluted from the core.

It is likely that this permeability damage resulted from swelling of the pore-lining clays.

(ii) NODO 1

The second plug had an initial average permeability to kerosine of 35 md at a pressure differential of 12.4 psi in.sup.-1. Its permeability increased gradually with increasing kerosine throughput. This trend was attributed to removal of residual crude oil from the rock matrix.

13.8 pore volumes of the NODO 1 oil based completion fluid, i.d. a fluid flux of 15.4 ml/cm.sup.2 of rock face, were injected through the core. The plug's permeability to kerosine at steady-state conditions increased to about 46 md after the treatment. As for the water based completion brine treatment, the plug permeability varied slightly after the treatment, depending upon fluid flow direction. In reverse flow the permeability was 46 md; in forward flow it was 42 md. The reason for this discrepancy is not clear at present. No fines were detected during the experiment.

The results of these tests are shown graphically in FIGS. 2 and 3 of the accompanying drawings wherein FIG. 2 shows how the permeability of the core is affected by the flow of completion brine and FIG. 3 by the flow of NODO 1.

The ZnBr.sub.2 /CaBr.sub.2 aqueous completion brine caused a substantial decrease in the permeability of the reservoir material to kerosine. In contract NODO 1 caused no damage to the core and actually slightly improved its permeability to kerosine.

Bromine (179.8g, 58.0ml, 4.5 equivalents, 1.125 mole) was added dropwise over 1.5 hours to a stirred suspension of ethylbenzene (26.50g, 0.25 mole), iron powder (2.60g) and carbon tetrachloride (50ml) at 8-10.degree. C. under nitrogen. After the addition was complete the mixture was stirred at 25-30.degree. C. for 1 hour and then slowly treated (with cooling and stirring) with aqueous sodium metabisulphite (0.63molar, 30ml). The organic layer was separated and washed with further aqueous metabisulphite (2.times.30ml), aqueous sodium carbonate (0.70 molar, 30ml) and water (2.times.50ml). The organic extract was then dried (MgSO.sub.4) and evaporated giving the brominated product as a mobile light yellow oil (89.3g, 77%).

The products were analysed for specific gravity, and bromine content by X-ray fluorescence and by 60 MH.sub.z.sup.1 H nuclear magnetic resonance. By calculating the ratio of aliphatic to aromatic protons via NMR integration a measure of the products' bromine content could be obtained. These values were in good agreement with the analytical figures. Results are set out in the following Table 5.

The general procedure of Example 5 was repeated at ambient temperature (10-16.degree. C.) and using less solvent (25ml).

Results are set out in the following Table 5.

The general procedure of Example 5 was repeated using a cumene feedstock. Detailed experimental conditions and results are set out in the following Table 5.

In both Examples 5 and 6 mobile liquid products of high density were obtained.

NMR studies (.sup.1 H and .sup.13 C) indicated the presence to two major isomers in Example 6, viz ##STR1## plus 3 further components. ##STR2##

Gas chromatography and mass spectral analysis indicated the following pattern of brominated products.

There was no evidence of side chain bromination.

A mobile liquid of high density was again obtained Example 7. NMR studies (.sup.1 H and .sup.13 C) indicated the following composition: ##STR4## GC/MS showed the following brominated products:

Again, there was no evidence of side-chain bromination from NMR or GC/MS.

The products of Examples 5-7 are suitable for use as well bore fluids, either neat or in diluted form to give a fluid with any desired density between that of the diluent itself and the brominated product, as the results in the following Table 6 show.

JP 5 is an odourless kerosine solvent, typically boiling in the range 190.degree. C.-255.degree. C., S.G. 0,785 and Pensky Martens flash point 72.degree. C.

The brominated cumene product was found to be a liquid across a wide temperature range (-20.degree. to 40.degree. C.), both neat and in 10% solution. Although the brominated ethylbenzene and its 10% solution were solid at -20.degree. C., this is a very severe test and pumpable fluids were obtained above -10.degree. C.