Influence of Drilling Fluid on the Collecting Properties

of Pay Beds

1. Learn the meaning of the following words, word-combinations and word groups:

futile, inferior, bulk, influx, contingent, pay area, well completion time, suspend, augment, differential, uphole velocity, permeability, fine-dispersed, cross-section, pervious, clog, fissure, scores, collecting properties of the sands, slurry, reservoir, connate, swell, saturation, elevated water abundance, emergence, biphasic, diminish, precipitation, foul.

2. Read and translate Text 1:

Text 1

The final aim of drilling oil and gas wells is to obtain the desired products (crude oil and gas). Great expenditures of labour, materials and money may prove futile if it becomes impossible to achieve the inflow of crude oil and gas; they will also appear in a large measure unjustifiable in the event of the crude oil and gas inflow being much inferior to the potentially possible one. The final result of drilling, e.g. the bulk of the influx and the well completion time, are contingent upon the quality of the drilling fluid used in drilling over a pay area, the method employed in approaching it and the way of completing the well.

Observations made in oil-fields over lengthy periods of time along with special long-term investigations have proven that the mud fluid is capable of exercising great influence on the well completion time, the labour-consuming nature of this operation and the size of the crude oil and gas influx (production rate). One may cite many examples when it proved impossible to obtain crude inflow from a well sunk with the application for its flushing of a chemically untreated clay mud, although the neighbouring wells put clown with other drilling fluids used for flushing showed fairly large yields.

There are known many instances when wells sunk at a high schedule speed with fresh water flushing were completed with considerable difficulty and their production rate was much lower than that of wells drilled with the use of a high-standard clay mud. Much time was spent in completing them, while the economy achieved thanks to a high drilling speed was lost completely or partially on account of exuberantly prolonged work of completion.

Not infrequent are cases when an intensive gassing of the mud or the appearance in it of oil are observed in the course of drilli ng. On the other hand, following cementation of the well and perforation of the casing it becomes impossible to obtain any inflow of gas (or crude oil) from the reservoir.

Quite often an abundant influx of crude oil or gas occurs in testing of the well. But if the operation of such a well has to be temporarily suspended, and for this purpose it is filled with a clay mud, on its recompletion the recovery becomes, as a rule, substan­tially lower and the inflow induction work greatly increases in length. Sometimes il becomes impossible to complete at all such a well.

What is then the reason for such a potent action produced by the quality of the mu d fluid on an effective opening of a pay bed?

While pay sands (beds) are being drilled over, a liquid phase is filtering out into it from the mud fluid. The volume of the filtrate entering the reservoir augments with increasing water loss of the mud, the time spent in drilling over the bed, the pressure differential, the uphole velocity in the annulus, the temperature of the mud, and with decreasing clearance between the drilling string and the borehole walls. The radius of the filtrate penetration into the pay bed may be as great as a few metres. The pay beds practically carry a certain amount of clayey and other particles, sensitive to the acti on of the filtrate and capable of interacting with it.

Another factor impairing permeability of collecting sands under the effcct of the mud fluid is penetration of fine-dispersed solid phase particles (clay particles and those of weighting material ) into the sands along large pores and microfractures, closu re of these pores or reduction of their effective cross-section.

The higher the permeability of the rock, the more, as a rule, the number of large pore channels therein. Therefore, a highly pervious rock is apt to be clogged to a greater extent with the solid phase particles than a little pervious one and the degree of its impaired permeability is greater. In a number of cases the permeability of sandstones, for instance, decreases for this reason as much as 10 and more times. The solid phase particles, apparently, do not enter at all into granulated collecting sands with a very low permeability.

It is not infrequent that in the co urse of dr illing the high pressure exerted by the column of mud fluid causes hyd raulic fracturing of the pay bed, or opening of natural, fractures therein. In this case the w hole of the mud fluid spreads about along the fissures and goes scores of metres deep down into the bed. Then, the collecting properties of the sands are liable to suffer much more badly. Of no less grave consequences is hydraulic fracturing occurring at the time of cementation and subsequent penetration along the fracturcs of the cement slurry and of its filtrate linto the reservoir.

Since a pay bed always carries a certain amount of connate water, the penetration of an aqueous filtrate into the reservoir gives rise to swelling of the clay particles and to a subsequent increase of water saturation. During completion of a we.il, oil and gas have to move toward the well via a zone around its be re characterized by an elevated water abundance. This results in the emergence of a biphasic flow (crude oil and water, or gas and water) and the effective permeability for the oil (gas) diminishes. The bulking up of argillaceous rocks, formation of insoluble salts and their precipitations, as well as that of resins, paraffins and other solid particles, into a sediment, consolidation of this sediment and clogging of the pore channels, all this, naturally, does not occur instantaneously. Therefore, the degree of damage due to the fouling of the reservoir with the mud fluid arid its filtrate is largely dependent upon the duration of their action: the longer it is, the greater the damage.