Ά wpirm OF nτspreτττ.π cw ΓRTTJ.THJG WASTES

This invention relates to a method of di-sposing of drilling wastes. More specifically, the invention relates to the treatment of formations around a hydrocarbon production area such as oil wells, gas wells, or similar hyό-πxarbon-^x>ntaining formations. It is particularly directed to disposal of drilling wastes produced during the drilling process.

In order to obtain hyά oc^bonaceous fluids from a productive interval of a formation, it is often necessary to drill wells so as to remove said fluids. During the drilling process, drilling fluids and drilling muds are used to facilitate the drilling process. These drilling fluids and muds are recovered and contain materials which may contaminate the environment if not disposed of properly. Conscientious efforts have been made to dispose of used drilling fluids, drilling muds, and related materials, in a manner that is as environmentally safe as possible. Sometimes the wastes are solidified in mud pits at the surface with cement slurries. Later, the slurries are covered with earth. Occasionally, the materials may be pumped into disposal wells or into a high permeability depleted oil interval of a formation. Disposal of drilling wastes offshore is particularly expensive because mud pits are non-existent and there may not be ready access to disposal wells. In various regions, tanks are used for storing drilling fluids and cuttings because reserve pits are not permitted. In addition, removal may be required from presently existing reserve pits. In such reserve pits, used drilling fluid contents are mixed with, or contaminated by, topsoil and the material used in constructing the pits.

Therefore, what is needed is a method to avoid the expense of hauling such materials from reserve pits or tanks to hazardous waste disposal locations which may be hundreds of miles from drilling sites.

Aαxϊrding to the present invention there is provided a method for disposing of drilling wastes cc-mprising:

(a) dispersing drilling wastes with an aqueous saline solution thereby forming a drilling waste and saline mixture;

(b) injecting said mixture into an underground formation which is of a coπposition sufficient to contain said mixture; and

(c) fracturing hydraulically said formation so as to allow continued injection of the drilling wastes at relatively low pressures into a created fracture in said formation.

Preferably the drilling waste comprises drilling fluids, drilled solids, drilling muds, or mixtures thereof.

Die mixture may be injected into said formation via at least one disposal well which penetrates said formation.

The mixture may be injected into a high permeability oil depleted zone of said formation. Preferably in step (a) the mixture contains a deflocculant which "shrinks¹¹ any hydrated bentonite particles entrained in said mixture which facilitates the disposal of mud solids. Preferably also, in step (a) the saline solution coπprises sodium chloride, potassium chloride, produced formation water, or mixtures thereof.

The hydraulic fracturing is desirably repeated whenever a "skin" builds υp due to filtration of solids within the formation so as to re-establish crmαanication with a disposal zone.

The mixture is desirably injected at fracturing pressures and rates into said underground formation.

The present invention provides a method of disposing of drilling wastes obtained while drilling holes into a formation for the recovery of hydrcicarbonaσeous fluids. The method is readily adaptable to various geographical areas, including offshore drilling sites.

In the practice of this invention, an aqueous slurry containing drilling wastes is prepared. The drilling wastes used herein are obtained as a result of the process of drilling holes into the earth in order to produce hycirocartoonaceous fluids therefrom. These drilling wastes are the result of circulating a drilling mud or drilling fluid down into a wellbαre productive tubing and up through an annulus during the drilling process to aid in cooling and lubricating a rotary bit. It is also used for providing a medium of lifting earthen material or so-called drilling cuttings out of the wellbore. The drilling fluid may coπprise water, certain clay-like materials to increase its viscosity and density, and other conditioning agents or additives.

Ordinarily, tanks σr reserve pits are used for storing the drilling fluids and αittings. Following a suitable separation process, the recovered drilling wastes are mixed into an aqueous saline solution. A suitable method for separating drilling wastes is discussed in US-A-4,942,929

An aqueous solution is utilized to prevent an uncontrolled migration of pre-existing formation ines into an area of lesser permeability. Fresh or relatively fresh water being foreign to the formation often causes any pre-existing quiescent fines to be dispersed from their repository or loosened from adhesion to capillary walls. If an abrupt decrease in salinity should occur, a large number of clay particles, or fines, can be released in a short time. This occϊirrence is avoided by the use of the saline solution herein. The effects of an abrupt decrease in salinity is discussed in US-A-4,570,710 After making a slurry of the drilling wastes which are of a size suf icient to fit into a predetermined more permeable zone in the formation, the slurry is injected into the formation at pressures sufficient to fracture the formation. High pressure hydraulic fracturing of the more permeable interval allows the drilling wastes to be continually injected into the formation at relatively low pressures. Fracturing can also be used to re-establish cxxtEnunication with a disposal zone whenever a "skin" has built up within the formation due to filtration of solids from the drilling wastes. A hydraulic fracturing technique which can be used herein is discussed in US-A-4,067,389. Another method for initiating hydraulic fracturing is disclosed in US-A-4,378,849. As is known by those skilled in the art, in order to initiate hydraulic fracturing in a formation, the hydraulic pressure applied must exceed the formation pressure in order to cause a fracture to form. The fracture which forms will generally run perpendicular to the lease principal stress in the formation or reservoir.

Salts which can be utilized in the saline solution include salts such as potassium chloride, magnesium chloride, calcium chloride, zinc chloride and carbonates thereof, preferably sodium chloride. Additionally, produced saline formation water can also be utilized. While injecting an aqueous salt or saline solution in a concentration sufficient to prevent fines migration, and sufficient drilling fluid wastes to make a slurry, pressures applied to the wellbore cause the aqueous saline solution to be farced deep within the formation. The depth to which the slurry is forced within the formation depends upon the pressure exerted, the permeability of the formation, and the characteristics of the formation as is known to those skilled in the art. In order to allow the fines or particles to migrate deeply within the formation, the critical fluid flow velocity of the slurrified disposal waste is exceeded. This causes the slurrified disposal waste to be transported in the saline solution to a location aep within the formation. Said slurry can be injected incrementally into an injection well where slugs ∞ntaining a higher cxmcentration of disposal fines in the slurry follow a slug of lower disposal waste concentration.

As used herein, the critical fluid flow velocity is defined as the smallest velocity of the saline solution which will allow fines or small particles to be carried by the fluid and transported within the formation or reservoir.

Lower velocities will not entrain particles and will permit particles to settle from the solution. A discussion of the saline concentration and the rate and velocity needed to dispose of the disposal waste is contained in US-A-4,787,452. A deflocculaπt can also be added to the saline solution.

The deflocculaπt is added in order to "shrink" hydrated beπtαnite particles to facilitate mud solids disposal. Deflocculants which can be used herein include floating carbonate, sodium hydroxide, and polyacrylic salts. Other deflocculants which can be used herein include calcium tolerant defloσculaπts which are discussed in US-A-4,770,795. Binary deflocculating oαmpositions are discussed in US-A-4,742,105. Hydrolized ∞polymers of n-vinylamide and acrylamide can also be utilized for deflocculants; this deflooculant is discussed in US-A-4,921,621. The slurrified drilling waste of this invention need not be injected coπt-inuously. A preferred method is to inject the slurrified drilling waste followed by a spacer volume of a saline solution. Once the slug of slurrified drilling wastes has reached the desired location, pressure is released which allows the fines to settle out and plug pares within the formation. This process can be repeated until the permeability of a formation has been decreased to the extent desired, or until it no longer will accept additional slurrified drilling waste.