A New Calcium-Tolerant Polymer Helps To Improve Drilling-Mud Performance and To Reduce Costs
- K.H.W. Ujma (Preussag A.G. Erdol und Erdgas) | J.P. Plank (SKW Trostberg A.G.)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling Engineering
- Publication Date
- March 1989
- Document Type
- Journal Paper
- 41 - 46
- 1989. Society of Petroleum Engineers
- 1.6 Drilling Operations, 1.11 Drilling Fluids and Materials, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 2.2.3 Fluid Loss Control, 4.1.2 Separation and Treating
- 2 in the last 30 days
- 274 since 2007
- Show more detail
- View rights & permissions
|SPE Member Price:||USD 5.00|
|SPE Non-Member Price:||USD 35.00|
Drilling fluids suitable for German Zechstein formation wells require fluid-loss polymers that tolerate up to 140,000 ppm of Ca/Mg and have temperature stabilities that exceed 350 degrees F [177 degrees C], Problems experienced in previous wells with carboxymethylcellulose (CMC) and polyanionic cellulose (PAC) are described. The use of starch with a new sulfonated polymer improved drilling mud performance and reduced costs. Major benefits come from a synergism of the new polymer with starch, resulting in a 50 degrees F [25 degree C] gain in starch thermal stability. A similar temperature extension effect was found for such cellulose products as CMC or PAC after addition of the polymer. Field experience with the starch/new-polymer combination in Zechstein formation wells with depths to 15,750 ft [4800 m] shows excellent filtration control, hardness tolerance, and temperature stability for this system. A cost analysis confirms saving of 20 to 50% on fluid-loss polymer costs.
Exploration for oil in West Germany has practically ceased because of depressed oil prices. Considerable gas drilling at depths between 10,000 and 20,000 ft [3000 and 6000 m], however, remains. Most of this deep exploration is done in northeastern and northern parts of West Germany, where huge salt structures are common. Among the different salt zones, the Zechstein formation is widely known for creating severe drilling problems because of its high content of divalent cations. The water-based drilling fluids used in such Zechstein formation wells often experienced serious performance problems or excessive costs.
In this paper, we report on new developments regarding improved fluid-loss-control additives used in deep Zechstein formation wells. The introduction of a new sulfonated polymer with exceptional NaCl and Ca tolerances greatly enhanced drilling-mud performance and reduced costs in such wells. The paper also presents basic field experience in German Zechstein formation drilling.
Zechstein Formation Drilling Problems
The Zechstein formation extends from northeastern Germany into Holland and the southern part of the North Sea (the U.K. and Denmark sectors). Besides NaCl, this zone often contains excessive amounts of Ca and Mg chlorides or sulfates, which cause most drilling problems. For example, up to 140,000 ppm of Ca/Mg hardness is found in a typical Zechstein brine, which is even worse than what operators in the Mobile Bay, U.S. A., area once saw. Table 1 shows the ion analysis for a typical Zechstein brine influx obtained 3 years ago in a German exploratory well.
Fluid-Loss Control. Experience has shown that fluid-loss-control additives to be used in Zechstein wells require (1) a tolerance to NaCl up to saturation; (2) an exceptional tolerance to divalent cations, notably Ca and Mg, up to 140,000 ppm of total hardness; (3) temperature stability from 200 to 400 degrees F [93 to 205 degrees C]; and (4) an absence of thinning or dispersing properties-e.g., known from lignites or lignosulfonates.
A common sequence of fluid-loss additives used lately in Zechstein formation drilling is cellulose-type polymers-e.g., technical grade CMC, PAC, hydroxyethylcellulose (HEC), or carboxymethyl HEC (CMHEC) up to 300 degrees F [150 degrees C], and vinylsulfonate/vinyl-amide (VS/VA) copolymer above 300 degrees F [150 degrees C]. This sequence has advantages and disadvantages.
The advantages of the most often used CMC- or PAC-type additives are the simple design and easy monitoring of the drilling mud. Once major calcium influxes occur, however, heavy bicarbonate treatment of the mud is necessary to precipitate the Ca and to maintain a proper CMC or PAC performance. Fig. 1 illustrates how technical-grade CMC is affected by Ca and Mg ions. In a detailed laboratory study, we found that in an unaged, salt-saturated, weighted mud, technical-grade CMC remains practically unaffected by Mg ions but deteriorates when Ca content exceeds 1,000 ppm. Note that in field practice even heavy bicarbonate treatments often did not yield satisfying results. Reasonable fluid-loss control usually could he achieved only with extremely high dosages (14 to 17 ppb [4 to 5 wt%]) of CMC. Furthermore. technical-grade CMC in salt-saturated, weighted muds showed thermal degradation starting at 290 degrees F [145 degrees C] (see Fig. 2).
For various reasons, the use of HEC or CMHEC as replacements for the less hardness-tolerant CMC- or PAC-type additives did not gain broad acceptance, although some operators tried it in Germany.
The thermal limit of cellulose-type polymers necessitated the use of the more temperature-stable VS/VA copolymers in a rather early stage of making the hole. In many cases, these copolymers provided sufficient Ca and temperature stability, but their major disadvantage was excessive cost. The expenses for this drilling fluid often were so high that we stocked used drilling fluid containing the VS/VA copolymer and reused it in other wells.
Proper fluid-loss control at reasonable costs in Zechstein formation wells was often unattainable. Therefore, Preussag A.G. encouraged mud service companies and chemical manufacturers to present improved products for this application.
Use of Starch. In previous years, starch was used infrequently in German drilling operations. It was known as a natural product subject to bacterial degradation with poor thermal stability. Starch also had the stigma of slowing rate of penetration (ROP) by some crosslinking effect with drill solids.
In a detailed laboratory study, we found that most of this negative reputation, at least for pregelatinized starch, was not true. Starch turned out to be a highly cost-effective fluid-loss material for high-salt and -hardness conditions that required no more handling precautions than other fluid-loss polymers. For example, in a salt-saturated mud, starch is temperature stable to - 240 degrees F - 115 degrees C]; in fresh-water muds, its temperature stability reaches 270 degrees F - 133 degrees C] (see Fig. 3). We also found that in salt-saturated muds, bacterial attack on starch never occurred because of the high salinity of the system. Because starch was completely Ca- and Mg-tolerant and was the most economical hardness-tolerant, fluid-loss polymer, we used pregelatinized starch in a number of wells and were successful, None of the disadvantages assigned to starch, like slowing of ROP, were experienced. Today we use starch as our standard fluid-loss additive for all high-salinity and high-hardness wells with temperatures to 240 degrees F [115 degrees C]. We think we have a technically superior and more cost-effective product compared with CMC or PAC in this temperature range.
Introduction of a New Polymer
After we established starch as the primary fluid-loss reducer in the lower-temperature range, a need for a NaCl- and Ca/Mg-tolerant, economic fluid-loss reducer for the temperature range of 240 to 400 degrees F [115 to 205 degrees C] still remained.
|File Size||459 KB||Number of Pages||6|