Drilling process using a shale protecting polymer drilling fluid system

Claim 2

The process of claim 1 in which said initially circulated clear fluid is a gas.

Claim 3

The process of claim 1 in which said initially circulated fluid is brine.

Claim 4

The process of claim 1 in which said initially circulated fluid is said shale-stabilizing aqueous solution of polymer and salt.

Claim 5

The process of claim 1 in which said initially circulated fast drilling fluid is gas and said subsequent circulations of (a) a shale-stabilizing aqueous solution of polymer and salt and (b) a suspension of prehydrated bentonite in that solution are effected substantially simultanteously by a circulation of said suspension.

Claim 6

A process for drilling a borehole that encounters a water-sensitive shale, with the borehole being drilled by operating a drilling means and circulating a drilling fluid, which process comprises: initially enhancing the rate of drilling by circulating as the drilling fluid a relatively low density, fast drilling, substantially clear fluid; when the encountering of borehole instability due to the interaction of water and water-sensitive shale is at least imminent, circulating as the drilling fluid an aqueous solution that (a) contains a partially hydrolyzed polyacrylamide which is about 20 to 50 percent hydrolyzed, has a molecular weight of more than about 1 million, and is present in an amount at least sufficient to satisfy the minimum adsorption requirements of the shale and (b) contains an amount of alkali metal halide which is at least sufficient to enhance the adsorption of polymer on the shale, with said aqueous solution being circulated so that the shale is contacted by the solution and stabilized by the adsorption of the polymer; and when the encountering of mechanical borehole instability due to a sloughing of mechanically incompetent earth formations is at least imminent, suspending a drilling fluid viscosifier in said aqueous solution in an amount that provides a suspension having both a relatively high solids-lifting capacity and a filter loss that is high enough to permit said shale stabilization, and circulating the suspension so that the earth formation solids are removed by the circulating suspension while said shale stabilization is continuing.

Claim 7

In such a use a small amount of alkali metal silicate and/or hydroxide can be added to the drilling fluid (such as sodium and/or potassium silicates and/or hydroxides). Potassium silicate or mixtures of potassium silicate and sodium hydroxide are preferred and the preferred and the amount used should be such as to maintain the pH of the total drilling fluid composition in the range of 9-12, preferably between 9.5 and 11.5. By the addition of 0.05-0.1 percent volume of 40* Be'' sodium silicate liquid or 0.05-0.1 percent weight metasilicate granulas and maintaining the pH between 9.5 and 10.0 with additional NaOH to a shale stabilizing drilling fluid of the present invention (such as an aqueous KCl-Separan AP-273 solution) corrosion can be effectively controlled or eliminated. It is additionally advantageous to use metasilicate when such a shale stabilizing solution is used for clear fluid drilling since the metasilicate tends to aid in the flocculation and settling of solids. However the silicate should not be used in waters, containing more than about 100 parts per million calcium ion since the silicate will be precipitated as in insoluble calcium silicate. The suspension of pre-hydrated bentonite in the aqueous solution of polymer salt is used when additional hole cleaning capacity is required. Field tests have indicated that in many drilling situations from about 4-6 pounds per barrel of prehydrated bentonite can readilby be suspended in such a solution to provide a stable mud system, particularly if an additional 1/4 per barrel of the polymer is added to compensate for the polymer adsorption by the bentonite. In such a mud the rheological properties tend to increase with increasing concentrations of salt until amounts in the order of 70 pounds per barrel are reached at which time the rheological properties tend to decrease with increasing concentration of salt. In general, the rheological properties of the present drilling fluid system can be varied from a clear fast drilling system to a gelled system with excellent hole cleaning properties. The viscosity of the system can be adjusted by dilution with fresh water or by the addition of pre-hydrated bentonite or viscosifiers such as the Kelzan XC polymer. Filter loss control to within 8 cc''s is achieved, for example, by addition of Drispac. Solids control is achieved by means of fluocculents in the clear fluid system and by the use of dilution/shakes/desilters in the gelled system. When the pre-hydrated bentonite is included, the API 30 minute filter loss is preferably maintained at from about 8 to 30 cc''s. The filter loss should be low enough to avoid a buildup of thick filter cakes along highly permeable earth formations that could cause tight hole problems. On the other hand, it should be high enough to avoid a reduction in the shale stabilizing effect of the polymer deposition from the aqueous solution filtrate. Drilling mud thinners and filter loss additives can be used where desirable Particularly suitable thinners are lignosulfates and particularly suitable filter loss additives are Dripac (polyanionic celluslose, Drilling Specialties Co.) and OC-Polymers (Xanthan bioppolymer, to 1/2 pound per barrel of either is generally sufficient. The filter loss agent should be utilized with caution to avoid reducing the API 30 minute filter loss to below about 8 ml. In order to obtain the present effective shale stabilization, the polymer must be a polyacrylamide that has shale protection properties and sufficient proportions of the polymer must be available to the shale to effect that protection. If the filter loss control is such that the mud provides a filter cake that is too tight, the amount of polymer that is available to the shale surfaces behind the filter cake may be decreased beyond the minimum required to stabilize the shale. Weighting materials can be used in the polymer muds of the present invention. Such materials can be conventional weighting materials such as barite, iron oxides, etc. It has been found that samples weighted to values in the order of 13.8 pounds per gallon maintain good rheological properties and shale protection. Since the partially hydrolyzed polyacrylamides such as Separan AP-273 is adsorbed on barite at the rate of about 0.03 pounds of polymer per 100 pounds of barite, an additIonal proportion of polymer (such as an additional 0.03 pound per 100 pounds of barite) should be added to compensate. The equipment utilized in drilling with the present clear fuild and subsequently bentonite-containing mud can be generally conventional equipment. As known to those skilled in the art, the bentonite is preferably prehydrated at the well site shortly before use and the prehydration system should be designed to prevent salt contamination from the main mud system. The use of a separate circulation system is advantageous and the prehydrating or preyielding tank can be independent or one compartment of a suction tank. Field Tests (Examples 42-44) The present polymer muds have a good tolerance for normally encountered drilling fluid contaminants and anhydrite stringers have been drilled during field tests without any detrimental effects other than the decrease in mud rheological properties that would be expected by those skilled in the art. More than 400 parts per million cacium ion has been found to cause the mud viscosity and yield to be descreased, however the changes were corrected by simply adding additional bentonite. Alternatively the calcium ions can be removed by adding barium carbonate or sodium carbonate if care is used with the latter to prevent the addition of an excess of the carbonate ions. In the following field tests, clear drilling fluid aqueous solutions of from about 0.3 to 0.6 pounds per barrel Separan AP-273, from about 10.5 to 35 pounds per barrel potassium chloride, and sufficient sodium hydroxide to maintain a pH of from 9.5 to 10 were used. And, such solution were converted to polymer muds by addition of from about 10 to 20 pounds per barrel prehydrated bentonite to the aqueous solutions. The maintenace of the polymer muds included adjustments of clay content or filter loss material as required to maintain and API 30 minute filter loss of from about 8 to 30 ml. Test example 42 was conducted in a borehole in which the upper portion was gas-drilled, from an intermediate casing at about 3, 600 fee to a depth of 6,000 feet, at which depth it became necessary to mud-up the hole due to sloughing and otherwise generally bad hole conditions. The dry gas-drilled hole was mudded-up with the above polymer mud with no problems. About 48 hours after the mud-up, earth formation cavings having diameters of from about 1/2 to 3/4 inch were filtered out of the returning mud stream by the shale shaker. This occurred for a period of about 10 to 12 hours after which little if any caving was apparent. The drilling with the polymer mud proceeded without incident to a depth of 10,263 feet at which point 7 5/8 inch intermediate casing was run. The casing was cemented and the bonding provided an excellent cement bond log. The polymer mud weight was controlled at between 8.8 and 8.9 pounds per gallon by using only the shale shakers and thedesilters to control the amount of suspended clay. While the mud viscosities were being maintained in the range of 100 to 150 seconds per quart, a fine screen high speed shaker was used. The viscosity control was achieved by simply adding pre-hydrated bentonite or diluting with water as required. Sour gas was encountered after round tripping the drill string from a dpeth of 7,100 feet. The mud blackened but no observable viscoity effects could be noted. Test example 43 was conducted in an area in which severe hole instability problems had been previously experienced at depths below 9,000 feet when drilled with conventional fresh water mud systems. No instability problems