Chrome free lignosulfonate are increasingly used in aqueous drilling fluids because of potential environmental concerns associated with the chromium counterparts. A basic design study of a chrome-free lignosulfonate showed three major reaction parameters affecting its performance: type and concentration of complexed metal ion. oxidation level under optimal pH and lignosulfonate raw liquor. A new chrome-free lignosulfonate formulation based on titanium metal ion was developed and its plant scale-up was conducted. Testing of the new titanium-complexed lignosulfonate was found to give improved rheological control vs. other available chrome-free lignosulfonates in various water-based drilling fluid applications.
Introduction
Chromium-based lignosulfonates were introduced to the drilling industry more than 30 years ago. They have been used as water-based mud additives continuously since initial introduction. During early development work with lignosulfonates, King described a drilling-fluid additive formulated with such metal ions as chromium, iron, and aluminum. and spent lignin liquor. The drilling industry has successfully used the chromium-based lignosulfonates as drilling fluid additives in a wide variety of drilling environments and over a broad temperature range.
Use of chromium lignosulfonate in drilling fluids has been discontinued in certain areas because of possible harm to the environment. It is generally accepted throughout the drilling industry that drilling fluid additives containing hexavalent chromium are toxic to the environment. Several papers have been published on the harmful effects of discharging fluids containing chromium or iron/ chromium lignosulfonates into waterways. If chromium-containing drilling fluids are used in a particularly sensitive area, it is necessary to remove the drilled cuttings from the site, dispose of the drilling fluid at a classified disposal site, and treat the holding pits used to contain such fluids. These environmental constraints have created the need for a chromium-free drilling fluid additive with processing-cost/performance characteristics and versatility equivalent to those of chromium lignosulfonates.
Several chromium-free modified lignosulfonate drilling fluid additives have been introduced to the drilling industry in recent years These chromium-free additives are, in many cases, not as effective as chromium lignosulfonate when tested in a wide range of applications. Some or all of the following performance deficiencies were observed when available chromium-free lignosulfonates were tested.
Typical drilling fluids treated with available chromium-free modified lignosulfonates proved less tolerant to solids and more sensitive to high temperatures than similar drilling fluids treated with chromium lignosulfonates. In addition, these chromium-free additives exhibited inferior performance when added to seawater and other inhibited drilling fluids. These fluids also displayed a lack of gel-strength control at high temperature and a sensitivity to pH variati.
A new chromium-free drilling fluid additive has been developed. This additive is a titanium-based lignosulfonate prepared under optimum reaction conditions. The titanium lignosulfonate controls rheological properties of drilling fluids as well as chromium lignosulfonates.
Experimental: Laboratory Synthesis of Chromium-Free Lignosulfonate.
A typical procedure for preparing a chromium-free drilling-fluid additive based on titanium lignosulfonate is described in this section.
Spent lignin liquor containing 58 wt% calcium lignosulfonate solids is diluted to 50 wt% solids. These solids are approximately 4 wt% calcium. The diluted solution is warmed to 120 deg.F [48.9 deg. C.
Calculated amounts of sulfuric acid and titanium sulfate solution are added to the calcium lignosulfonate solution with stirring. The precipitated calcium sulfate is removed by filtration.
A calculated amount of hydrogen peroxide is added to the filtrate at a controlled rate with a syringe pump. Temperature and pH changes are monitored during the peroxide addition. After oxidation is complete, the solution is cooled to 120 deg. F [48.9 deg. C] and then partially neutralized to a pH of about 3.5 with a 50 % sodium hydroxide solution. The solution is spray dried to obtain the final product as a dry powder.
Iron sulfate or zirconium sulfate can be used in conjunction with the titanium sulfate to prepare mixed-metal lignosulfonate salts.
Evaluation of Lignosulfonates
Preparation of Test Mud. A 12-lbm/gal [ 1437.9-kg/m ] fresh- water mud was prepared as follows and used as the test mud. The composition of 1 bbl [0. 16 m ] equivalent was 312.6 cm tap water, 18.0 g sodium smectite, 20.0 g calcium smectite, and 198.0 g barite. This test mud was aged 24 hours at room temperature before use.
Test-Mud Treatment. Individual barrel equivalents of the test mud were treated with calculated amounts of each modified lignosulfonate sample. The pH of these treated muds was adjusted to 11.0 with a 50 wt% sodium hydroxide solution. Selected contaminants were added to some of the muds to study the lignosulfonates’ ability to protect against contamination. Muds were aged for 16 hours in a rotating oven at selected temperatures.
Rheological Measurements. Apparent viscosity. plastic viscosity, yield point. gel strength, and fluid loss of the aged muds were measured according to API Procedure No, 13B using a Fann Model 35 VG meter. The high-temperature/high-viscosity relationship was measured with a Fann Model 50C viscometer. The Model 50C recorded viscosity changes of test muds during heating and cooling cycles. The heating rate was 3.8 deg. F/min [2.1 deg. C/min] to an upper limit of 450 deg. F [232.2 deg. C]. Pressure during the heating and cooling cycle was maintained between 500 and 800 psi [3447 and 5516 kPa].
The definitions and an explanation of these rheological properties and their measurement are discussed comprehensively by Gray et al.
Results
During research on lignosulfonates, King et al. showed that converting lignosulfonic acid to the chromium, iron, or aluminum salt and then oxidizing the salt yielded an effective drilling-fluid additive.
