Waters of Producing Fields in the Rocky Mountain Region By JAMES G. CRAWFORD,' MDlBER AHlIE (Denver ~jeeting, September 1947) ABSTRACT is tics peculiar to each field, each subsurface zone and each province. Criteria useful in CORRELATION of water with its reservoir zone or formation has been one of the applica­ postulating the occurrence of oil or gas in tions of oil-field water analysis of greatest direct one province completely fail when applied value to the petroleum engineer. The water in to another, and data carefully prepared and each producing zone tends to have diagnostic analyzed from one field may be actually characteristics by which it can be distinguished misleading when applied to another. from every other water above or below that Correlation of water with its reservoir zone in that immediate vicinity. Representative zone or formation has been one of the Downloaded from http://onepetro.org/TRANS/article-pdf/179/01/264/2178839/spe-949264-g.pdf by guest on 26 September 2021 analyses of oil-field waters from producing oil applications of oil-field water analysis of and gas fields in the Rocky Mountain region greatest direct value to the engineer. The are included, and the diagnostic characteristics concentrations and characteristics of the are discussed briefly. It is concluded that the generally dilute nature of Rocky Mountain oil­ waters are essential to the engineers and field waters is a result of dilution by meteoric geologists making interpretations of elec­ waters, and that there is no relationship be­ tric logs in this region. The behavior of tween presence or absence of commercial oil water under conditions of reservoir tem­ and the character of water in a structure. perature and pressure is dependent to a great degree upon its concentration and INTRODUCTION characteristics, and thus important to the The study of waters associated with oil reservoir engineer. With the advent of and gas began more than 50 years ago and secondary recovery methods in the Rocky has been well recognized by operators, Mountain region the characteristics and engineers, and geologists for about 30 treatment of water will become increasingly years. It appears unnecessary at this time important. to recite the history of oil-field water analysis; suffice to say that it has proved PREMISES UPON WHICH CORRELATION Is its worth many times to the production BASED engineer suddenly confronted with water A brief review of the geochemical history problems in producing oil and gas wells. of oil-field waters will suffice to present the Chemists and geologists have studied premises upon which correlations of the the possible origin of these waters and as waters are based. Sedimentary rocks which yet the subject is unsolved in many are now stratified were first sediments in important phases. There are both local and seas, lakes and streams. These sediments regional problems connected with the ex­ were filled interstitially with connate planation of concentrations and character- water. With burial the sediments were com­ pressed and consolidated, integrated and Manuscript received by the office of the Institute Sept. I, 1947. Issued as TP 238,3 in indurated into bed rock and much of the PETROLEUM TECHNOLOGY. May 1948. connate water was dispelled. Following the • Chemical Engineer, Chemical Laboratories, Inc., Casper, Wyo. formation of bed rock the strata were sub- 264 JAMES G. CRAWFORD jected to many changes, such as very deep above or below that zone in that immedi­ burial at zones of moderately high tem­ ate vicinity. The soundness of the premise peratures, and tilting and uplifting with is attested by the experience of the author erosion along exposed scarps, valleys and who has not found a violation in more than canyons. Exposed, eroded porous strata I8 years' work of correlating waters in the were invaded by ground water of meteoric Rocky Mountain region. origin whereby connate water has been displaced and diluted. Contiguous to ALTERATIONS SUBSEQUENT TO deeply incised streams under conditions of ENTRAPMENT arid climate the ground water may have It is doubtful if there has been appreci­ been lowered whereby connate water was able change in chemical characteristics of eliminated from strata several thousand water trapped in structures devoid of feet thick. These geologic and geochemical hydrocarbons. It is assumed that ground changes were slow and gradual over millions waters cannot infiltrate to any extent and of years. As a result the composition of that chemical changes after accumulation Downloaded from http://onepetro.org/TRANS/article-pdf/179/01/264/2178839/spe-949264-g.pdf by guest on 26 September 2021 water from a given zone appears very are negligible. In structures containing oil nearly constant throughout the economic or gas, though, a different picture is life of an oil or gas field. The amount of presented. Connate water has had the . connate water in a given bed and the opportunity of reacting with hydrocarbons amount of interstitial water subsequently and there is no doubt that alteration has migrated into a given bed provide the con­ occurred in many instances. The reactions centrations and characteristics peculiar to between hydrocarbons and different types each locality or geological province. of water are not thoroughly understood, The wide range in connate waters can but it is very well established that sulphate be shown by reciting that some brines in oil reduction, under certain conditions, does fields of other regions have concentrations take place. The equation involving this nine times that of normal sea water of reaction is 35,000 ppm total solids. In the Rocky Mountain region the greatest concentra­ tion in producing fields is IOO,OOO to I50,- It is thought that the reaction is aided or 000 ppm occurring in the Weber sand at initiated by anaerobic bacteria. The Rangely; the most dilute is 200 ppm total products of the reaction are both chemically solids in the Tensleep sand at Dallas, active; the sulphides may be precipitated Derby, Lander and Black Mountain. The from the solution as metallic salts and be­ concentration and chemical character­ come part of the strata; the carbon dioxide istics of a connate water may be more may escape as a gas, or become fixed in the nearly its original properties than has often aqueous solution as a bicarbonate. Pre­ been considered in earlier studies. The sea sumably this reaction would result in a water may have been modified by meteoric water containing little, if any, sulphate and waters at the time deposition was oc­ an excess of bicarbonate. curring. Thus, variations in oil-field waters The occurrence of hydrogen sulphide in do not always indicate alterations subse­ the oils and waters of pre-Triassic strata is quent to origin, and low concentration does striking. These waters occur in, or contact, not always indicate hydraulic flushing or limestone beds and are for the most part ground water infiltration. sulphate waters. It is believed that associ­ The important point is that water con­ ation with hydrocarbons has resulted in tained in each producing zone tends to have partial reduction of the sulphates in the diagnostic characteristics by which it can water, and the fact that the oil and water be distinguished from every other water both contain hydrogen sulphide gas is some 266 WATERS OF PRODUCING FIELDS IN THE ROCKY MOUNTAIN REGION evidence that active reduction is occurring the three oil sands, other water-bearing at the present time. It is noticeable that lenticular sand bodies. The operators have waters of these older formations in barren been conscientious about testing every structures often carry small to moderate sand body and obtaining water samples in amounts of hydrogen sulphide, the reduc­ an effort to correlate these lenticular sands tion being accomplished by carbonaceous from well to well. The analyses of these matter in the stratum. waters are extremely variable. They have The pre-Triassic waters do not contain concentrations ranging from 1500 to much alkalinity and it was difficult to 32,000 ppm total solids and a chloride con­ accept active reduction in these waters tent varying between 65 and 20,000 ppm. until one could account for the carbon The water is saline, the salinity being dioxide liberated by the reaction. For every caused almost entirely by chloride. The mol of sulphate reduced two mols of carbon erratic nature of these waters points to the dioxide are formed, and one would expect lack of continuity in the sands. Three a high alkalinity in the waters in which this typical analyses of Wasatch water in this Downloaded from http://onepetro.org/TRANS/article-pdf/179/01/264/2178839/spe-949264-g.pdf by guest on 26 September 2021 reaction occurred. Recent work on the field are given in Table I; No. I is from the natural gases of this region by the author first oil-producing sand. has shown that the older formations quite TABLE I-Typical Analyses oj Wasatch often yield gases containing notable quan­ Water, Hiawatha Field tities of carbon dioxide, and the source might well be sulphate reduction. No. I No.2 NO.3 OIL-FIELD WATERS OF COLORADO Na ........... 1.751 5.873 10.594 Ca ........... 94 30 903 Colorado has produced only about 6 pct Mg .......... 40 44 409 S04 .......... 867 18 162 of the total oil in the Rocky Mountain Cl. .......... 2,009 7.889 18.939 region but with the recent development of CO, .......... o 146 o HCO, ........ 575 2,005 115 T.S .......... 5,044 14.987 3 1 .064 the Weber formation in the Rangely field pH .......... the state is occupying a more important Source ....... DST Bailer Storage tank Depth ........ 2.234-2.287 3.350 2.3 10-2,354 place in oil production.