Earth boring – well treating – and oil field chemistry – Earth boring – Contains inorganic component other than water or clay
Reexamination Certificate
1999-09-10
2003-07-01
Tucker, Philip (Department: 1712)
Earth boring, well treating, and oil field chemistry
Earth boring
Contains inorganic component other than water or clay
C507S100000, C507S200000, C507S269000, C507S906000, C507S128000, C507S135000, C507S138000, C507S235000, C507S259000, C507S265000
Reexamination Certificate
active
06586372
ABSTRACT:
This invention relates to products which increase the density of wellbore fluids used during the construction or repair of oil, gas, injection, water or geothermal wells. The products of this invention may be used in any wellbore fluid such as drilling, cementing, completion, packing, work-over (repairing), stimulation, well killing, and spacer fluids.
One of the most important functions of a wellbore fluid is to contribute to the stability of the well bore, and control the flow of gas, oil or water from the pores of the formation in order to prevent, for example, the flow or blow out of formation fluids or the collapse of pressured earth formations. The column of fluid in the hole exerts a hydrostatic pressure proportional to the depth of the hole and the density of the fluid. High pressure formations may require a fluid with a specific gravity of up to 3.0.
A variety of materials are presently used to increase the density of wellbore fluids. These include dissolved salts such as sodium chloride, calcium chloride and calcium bromide. Alternatively powdered minerals such as barytes, calcite and hematite are added to a fluid to form a suspension of increased density. It is also known to utilise finely divided metal such as iron as a weight material. In this connection, PCT Patent Application WO85/05118 discloses a drilling fluid where the weight material includes iron/steel ball-shaped particles having a diameter less than 250 &mgr;m and preferentially between 15 and 75 &mgr;m. It has also been proposed to use calcium or iron carbonate (see for example U.S. Pat. No. 4,217,229).
It is a requirement of wellbore fluids that the particles form a stable suspension, and do not readily settle out. A second requirement is that the suspension should exhibit a low viscosity in order to facilitate pumping and to minimise the generation of high pressures. Another requirement is that the wellbore fluid slurry should exhibit low filtration rates (fluid loss).
Conventional weighting agents such as powdered barytes (“barite”) exhibit an average particle diameter (d
50
) in the range of 10-30 &mgr;m. To suspend these materials adequately requires the addition of a gellant such as bentonite for water based fluids, or organically modified bentonite for oil based fluids. A soluble polymer viscosifier such as xanthan gum may be also added to slow the rate of the sedimentation of the weighting agent. However, a penalty is paid in that as more gellant is added to increase the suspension stability, the fluid viscosity (plastic viscosity) increases undesirably resulting in reduced pumpability. This is obviously also the case if a viscosifier is used.
The sedimentation (or “sag”) of particulate weighting agents becomes more critical in wellbores drilled at high angles from the vertical, in that sag of, for example, one inch (2.54 cm) can result in a continuous column of reduced density fluid along the upper portion of the wellbore wall. Such high angle wells are frequently drilled over large distances in order to access, for example, remote portions of an oil reservoir. In this case it becomes even more critical to minimise a drilling fluid's plastic viscosity in order to reduce the pressure losses over the borehole length.
This is no less important in deep high pressure wells where high density wellbore fluids are required. High viscosities can result in an increase in pressure at the bottom of the hole under pumping conditions. This increase in “Equivalent Circulating Density” can result in opening fractures in the formation, and serious losses of the wellbore fluid. Again, however, the stability of the suspension is important in order to maintain the hydrostatic head to avoid a blow out. The two objectives of low viscosity plus minimal sag of weighting material can be difficult to reconcile.
The need therefore exists for materials to increase fluid density which simultaneously provide improved suspension stability and less viscosity increase.
It is known that reduced particle sedimentation rates can be obtained by reducing the particle size used.
However, the conventional view in the drilling industry is that reducing the particle size causes an undesirable increase in viscosity. This is supposed to be caused by an increase in the surface area of the particles causing increased adsorption of water.
For example, “Drilling and Drilling Fluids” Chilingarian G. V. and Vorabutor P. 1981, pages 441-444 states: “The difference in results (i.e. increase in plastic viscosity) when particle size is varied in a mud slurry is primarily due to magnitude of the surface area, which determines the degree of adsorption (tying up) of water. More water is adsorbed with increasing area.” Further it is also stated that “Viscosity considerations often will not permit the addition of any more of the colloidal solids necessary to control filtration, unless the total solids surface area is first reduced by removing a portion of the existing clays”. The main thrust of the argument is that colloidal fines due to their nature of having a high surface area to volume ratio will adsorb significantly more water and so decrease the fluidity of the mud. This is why they and others have recommended that it is necessary in weighted particulate muds to remove the fine solids to reduce viscosity. The same argument or concept is presented in “Drilling Practices Manual” edited by Moore pages 185-189 (1986). Also, the API specification for barite as a drilling fluid additive limits the % w/w below 6 &mgr;m to 30% maximum in order to minimise viscosity increases.
It is therefore very surprising that the products of this invention, which comprise particles very finely ground to an average particle diameter (d
50
) of less than two &mgr;mmicrons, provide wellbore fluids of reduced plastic viscosity whilst greatly reducing sedimentation or sag.
The additives of this invention comprise dispersed solid colloidal particles with a weight average particle diameter (d
50
) of less than 2 &mgr;m and a defloculating agent or dispersant. The fine particle size will generate suspensions or slurries that will show a reduced tendency to sediment or sag, whilst the dispersant controlling the inter-particle interactions will produce lower rheological profiles. It is the combination of fine particle size and control of colloidal interactions that reconciles the two objectives of lower viscosity and minimal sag.
It is worth noting that small particles have already been used in drilling fluids but for a totally different purpose. Thus, EP-A-119 745 describes an ultra high density fluid for blow-out prevention comprised of water, a first and possibly second weighting agent and a gellant made of fine particles (average diameter from 0.5 to 10 &mgr;m). The gelling agent particles are small enough to impart a good static gel strength to the fluid by virtue of the interparticle attractive forces. On the contrary, the present invention makes use of well dispersed particles: the interparticle forces tend to push away the other particles. If the concentration of small dispersed particles is sufficient, no gelling agent is needed.
According to the invention, a dispersant is added to the particulate weighting additive to allow it to find an acceptable conformation on the particle surface. This provides via a manipulation of the colloidal interactions rheological control, tolerance to contaminants and manipulation of the colloidal interactions rheological control, tolerance to contaminants and enhanced fluid loss (filtration) properties. In the absence of the dispersant a concentrated slurry of these small particles, would be an unpumpable paste or gel. According to a preferred embodiment of the present invention, the dispersant is added during the grinding or comminution process. This provides an advantageous improvement in the state of dispersion of the particles compared to post addition of the dispersant to fine particles. The presence of the dispersant in the comminution process yields discrete particles which can form a more efficiently packed filter cake and
Bradbury Andrew
Sawdon Christopher A.
Cagle Stephen H.
Howrey Simon Arnold & White , LLP
M-I LLC.
Tucker Philip
White Carter J.
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