The invention relates to drilling fluids and, more particularly, to a drilling fluid with an additive which acts as a thinner and conditioning agent to reduce viscosity, yield point and gel strength of the drilling fluid as desired.
Drilling fluids are widely used for drilling operations to create subterranean wells, for example from a surface location to a subterranean formation which may bear hydrocarbons to be produced through the well. During the course of drilling, a drill bit is rotated in the well bore and drilling fluids are circulated through the well to lubricate the drill string and the drill bit and also to carry cuttings from the formation at the location of the drill bit to the surface. In addition, the drilling fluid can serve to balance hydrostatic pressure encountered in various formations during drilling and serves numerous other purposes.
Thinners are frequently used in water-based drilling fluids for reducing their viscosity, yield point and gel strength when needed. Such thinners include chemical treatments with substances such as lignites, lignosulfonates, and modified tannins. These thinners generally have, as a common characteristic, an anionic charge. These anions are attracted on the edge surfaces of clay particles and neutralize charges and electronically compensate for the attractive charges between the particles, thereby decreasing the tendency of the fluid to form a gel structure, and decreasing plastic viscosity.
The modified tannins frequently used are from sources including Quebracho, mimosa, baloneea, and chestnut, and these modified tannins are effective deflocculants of water-based drilling fluids. These modified tannins are effective even when the drilling fluid is contaminated with other substances such as gypsum, cement, lime and salts, as these contaminants typically cause problems with respect to viscosity, yield point and gel strength. Unfortunately, the modified tannins referenced above, although widely used, have environmentally hazardous effects. The typical modifications involve modification with heavy metals such as chrome-tannates, fero-tannates, and stannus-tannates in order to improve their performance as a deflocculant at higher temperatures, and to provide more tolerance to salt and calcium contamination. However, none of these materials is environmentally acceptable.
It is clear that the need exists for a suitable additive for a drilling fluid which addresses the various issues with respect to viscosity, yield point and gel strength of the drilling fluid without causing the aforesaid environmentally hazardous effects. It is therefore the goal of the present invention to provide an additive for drilling fluids and a conditioned drilling fluid containing the additive which is effective at controlling viscosity, yield point and gel strength while also being environmentally friendly.
Other objects and advantages will appear hereinbelow.
In accordance with the present invention, the foregoing has been attained. Specifically, an additive for drilling fluids is provided which is based on unmodified tannins obtained from Caesalpinia coriaria or C. coriaria (divi-divi) which is a native Leguminosae widely distributed throughout Venezuela. A dry fraction of this particular species, namely a dry fraction extracted from the fruit pods of C. Coriaria, has shown to have excellent properties as a deflocculant and conditioning agent in water-based drilling fluids. This vegetable tannin has a very high content of hydrolysable tannins.
Therefore, in accordance with the present invention, a conditioned water-based drilling fluid is provided which comprises water-based drilling fluid and unmodified vegetable tannin comprising hydrolysable tannins.
In further accordance with the invention a method for drilling a subterranean well is provided, comprising the steps of: operating a drill bit in a well in the presence of a conditioned, water-based drilling fluid, wherein the drilling fluid comprises water-based drilling fluid, and unmodified vegetable tannin comprising hydrolysable tannins.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.
A detailed description of preferred embodiments of the present invention follows, with reference to the attached drawings, wherein:
The invention relates to drilling fluids and an additive for conditioning a drilling fluid for adjusting and generally reducing the viscosity, yield point and gel strength of the drilling fluids. In accordance with the present invention, the additive is an unmodified tannin obtained from the tannin fruit pods of C. Coriaria (divi-divi). These tannins have chemical characteristics that do not require modification to produce excellent performance as deflocculants and the process for extracting these tannins is a dry process. The tannic acid contained in the tannins is a particularly important aspect, particularly tannic acid which is classified as hydrolysable tannins, such as gallotannins and ellagitannins. Hydrolysable tannins have been found to be abundant in tannins obtained from the fruit pods of C. Coriaria. Spectrophotometric UV-visible analysis indicates a tannins content from C. Coriaria is about 65-70% as tannic acid. This tannic acid is classified as hydrolysable tannins as discussed above.
The unmodified tannins in accordance with the present invention are found to be an extremely useful additive to water-based drilling fluid to produce conditioned drilling fluid which has desirable properties with respect to reduction of viscosity, yield point and gel strength, all as will be further demonstrated further below.
As mentioned above, the drilling fluid additive of the present invention in the form of unmodified tannins extracted from the fruit pods of C. Coriaria produces excellent results in reduction of viscosity, yield point and gel strength of the drilling fluid, and is far more environmentally friendly than the typical modified tannins as discussed above.
As mentioned previously, the tannic acid content of the tannins obtained from C. Coriaria is a particularly advantageous and desirable quality of these specific tannins.
The dry process for extracting tannins from C. Coriaria fruit pods also involves separation of the divi-divi tannins from the various dry process materials, and grain size distribution has been found according to the invention to be of critical importance in obtaining suitable tannins. Specifically, following a typical dry process, the size range of between 15 μm and 150 μm, preferably between 45 μm and 75 μm, produces a fraction of the dried grain materials having the highest content of hydrolysable tannins.
FT-IR spectra of tannins were taken specifically of the divi-divi tannins, commercially modified tannins and straight tannic acid, and these spectra are illustrated in
The similarity between divi-divi tannins and tannic acid as shown in
Table 1 shows that the divi-divi tannins in accordance with the present invention presented properties as good as or better than the reference modified tannins, and this is significant given that the unmodified tannins of the present invention do not pose the same environmental risks as do the modified reference tannins.
In order to evaluate the divi-divi tannins in accordance with the present invention as a deflocculant, a number of water-based drilling fluids were prepared and then contaminated with a constant amount of gypsum. Yield point and gel values were tested, and these values increased considerably upon contamination with gypsum except for the fluids which were also treated with 1.5 and 3.0 pounds per barrel (ppb) of the divi-divi tannin. Tables 2 and 3 set forth the results of this testing.
Contamination of the fluids with gypsum created fluid flocculation conditions without needing to increase the pH, which helps to better study the performance of a system which might otherwise be sensitive to changes in pH, by creating a single variable, namely, deflocculation and not the pH.
In the formulation used for these tests, the reference material was a commercial modified tannin DESCO from Drilling Specialties Company, having deflocculant functionality as recognized in commercial operations with water-based drilling fluids.
Table 2 above shows water based drilling fluid contaminated with 1 ppb of gypsum, and the same base fluid treated with 0, 0.5, 1.0, 1.5 and 3.0 ppb of divi-divi tannin. Rheological readings are also presented initially and after aging at 16 hours and a temperature of 150° F., and excellent rheological properties are exhibited even after contamination with 1.0 ppb of gypsum, when 1.5 and 3.0 ppb divi-divi tannins are also included.
Table 3 shows that performance of the reference tannin with respect to a gypsum-contaminated drilling fluid was not nearly as effective.
The rheological behavior in accordance with the present invention was also evaluated in its reaction to different shear rates both before and after aging. The same base fluid discussed above was evaluated when contaminated with 1 ppb of gypsum and varying amounts of divi-divi tannin. The results of this testing are presented in
Referring to
Referring now to
With respect to yield point and plastic viscosity, these values were also taken both before and after aging for fluids treated with divi-divi tannins in accordance with the present invention and for fluids treated with a reference tannin. The results of this testing are presented in
The same values were taken after aging for 16 hours at 150° F., and the results of these tests are shown in
Next, in further evaluating the fluid in accordance with the present invention, the shear strength was measured at 10 seconds and 10 minutes for the base fluid, contaminated base fluid and formulations F2-F6 prepared each with divi-divi tannins in accordance with the present invention and reference tannins, and the results of this testing are shown in
Finally, the shear strength was also evaluated after aging at 16 hours at 150° F., and the results of the aging are shown in
Based upon the foregoing, it is believed to be clear that a drilling fluid containing at least about 1.0 ppb of divi-divi tannins in accordance with the present invention has substantially improved properties on all parameters measured and evaluated. It is believed that this is due to divi-divi tannin acting as a deflocculant through positive redox potential, unlike the reference tannin (commercial modified tannin) operating at negative redox potential. However, both load balancing mechanisms are explicable on the model of the diffused double layer. Water-based fluids contaminated with plaster or cement containing tannin divi-divi exhibit better before and after aging deflocculant performance with an increasing amount of tannin divi-divi, between 1.0-3.0 ppb, accentuating the rheological behavior with aging. The upper limit is 3.0 ppb, set by load balancing mechanisms.
The process of obtaining the divi-divi tannin which are desired according to the invention begins with one step cleaning and drying of the fruit. Then follows a stage of milling and sieving between 325 to 100 mesh, corresponding to the particle size of 45 μ to 150 μ. The tannin powder obtained is dried in an oven at 50° C.
Through ISO 9648:1998 for the determination of soluble tannin content in sorghum, the percentage of hydrolysable tannins in the tannin extract of the fruit from divi-divi was examined. The modification of the analytical procedure consisted basically of replacing other chemical reagents that meet the same analytic functions. Then, standard solutions of tannic acid were prepared at different concentrations, and a calibration curve absorbance versus concentration of tannic acid by UV-Visible spectrophotometer was prepared at a wavelength of 525 nm in the absorbance of the samples, transferred to the standard curve and the concentration of hydrolysable tannins and tannic acid was determined.
The addition of divi-divi tannin to drilling fluid according to the invention can be done in two ways, prevention and treatment. At the preventive stage, the addition of tannin to the drilling fluid is preferably lower, for example 0.5 pounds per barrel. In the treatment stage, depending on the concentration of the contaminant, between 1.0 and 3.0 pounds per barrel can be added. All alkaline pH of 9.5-10 is preferred.
It should be appreciated that a detailed description of preferred embodiments of the present invention has been presented above. This is to be viewed as illustrative of several embodiments of the present invention but not limiting upon the actual scope of the invention, which is instead measured by the scope of the claims set forth below.