CO2 Reserve Research Articles

Treatment of Central Sleep Apnea in US Veterans

There are no standard therapies for the management of central sleep apnea (CSA). Either positive pressure therapy (PAP) or supplemental oxygen (O(2)) may stabilize respiration in CSA by reducing ventilatory chemoresponsiveness. Additionally, increasing opioid use and the presence of comorbid conditions in US veterans necessitates investigations into alternative titration protocols to treat CSA. The goal was to report on the effectiveness of titration with PAP, used alone or in conjunction with O(2), for the management of CSA associated with varying comorbidities and opioid use. This was a retrospective chart review over 3 years, performed at a VA sleep disorders center. The effects of CPAP, CPAP+O(2), and BPAP+O(2), used in a step-wise titration protocol, on consecutive patients diagnosed with CSA were studied. CSA was diagnosed in 162 patients. The protocol was effective in eliminating CSA (CAI ≤ 5/h) in 84% of patients. CPAP was effective in 48%, while CPAP+O(2) combination was effective in an additional 25%, and BPAP+O(2) in 11%. The remaining 16% were non-responders. Forty-seven patients (29%) were on prescribed opioid therapy for chronic pain, in whom CPAP, CPAP+O(2), or BPAP+O(2) eliminated CSA in 54%, 28%, and 10% cases, respectively. CPAP, CPAP+O(2), and BPAP+O(2) each produced significant declines in the AHI, CAI, and arousal index, and an increase in the SpO(2). The data demonstrate that using a titration protocol with CPAP and then PAP with O(2) effectively eliminates CSA in individuals with underlying comorbid conditions and prescription opioid use. Comparative studies with other therapeutic modalities are required.

Effects of clonidine on breathing during sleep and susceptibility to central apnoea

We hypothesized that administration of clonidine would decrease the hypocapnic apnoeic threshold (HAT) and widen the CO2 reserve during non-REM sleep. MethodsTen healthy subjects (4 females) (age 22.3±3.0 years; BMI 25.5±3.4kg/m2) were randomized to receive placebo or 0.1mg/45kg of clonidine on 2 separate nights. Ventilation and upper airway resistance were monitored during wakefulness and sleep. Two separate experiments were performed: Protocol 1 (n=8), CO2 reserve, HAT and HcVR were determined using non-invasive hyperventilation (NIV) to induce hypocapnia for at least 3min; Protocol 2 (n=6), peripheral hypocapnic ventilatory response (HcVR) was determined by NIV using short (3 breaths) hyperventilation. ResultsClonidine decreased the systolic blood pressure by 12±10mmHg but did not affect baseline ventilation or upper airway resistance during wakefulness or sleep. Protocol (1), clonidine was associated with decreased HAT relative to placebo (37.3±3.3mmHg vs. 39.7±3.4mmHg, P<0.05), increased CO2 reserve (−3.8±1.3mmHg vs. −2.8±1.2mmHg, P<0.05), and decreased HcVR (1.6±0.6L/min/mmHg vs. 2.5±1.3L/min/mmHg, P<0.05). Protocol (2), administration of clonidine did not decrease peripheral HcVR compared to placebo (0.5±0.3L/min/mmHg vs. 0.7±0.3L/min/mmHg, P=NS). ConclusionClonidine is associated with diminished susceptibility to hypocapnic central apnoea without significant effect on ventilation or upper airway mechanics. Reduced susceptibility to hypocapnic central apnoea is not explained by the peripheral chemoreceptor pathway. This suggests a central rather than a peripheral effect of clonidine on the susceptibility to hypocapnic central apnoea.

Redox regulation of neurovascular function by acetazolamide: complementary insight into mechanisms underlying high‐altitude acclimatisation

We read with interest the study by Fan and colleagues describing how the carbonic anhydrase (CA) inhibitor acetazolamide reduced periodic breathing and accelerated acclimatisation to high altitude in native lowlanders (Fan et al. 2012). Intravenous infusion was shown to increase resting cerebral blood flow (CBF) and cerebrovascular reactivity to carbon dioxide () independently of altered peripheral or central chemoreflex sensitivity. The authors concluded that the improvement in breathing stability was attributable to an increase in the ‘CO2 reserve’. We would like to extend their elegant findings by suggesting a complementary mechanism albeit secondary to classical CA inhibition that may have equally contributed to the neurovascular benefits as observed.

The Emergence of Central Sleep Apnea after Surgical Relief of Nasal Obstruction in Obstructive Sleep Apnea

By the current definition, complex sleep apnea (CompSA) refers to the emergence of central sleep apnea (CSA) during the treatment of obstructive sleep apnea (OSA) with continuous positive airway pressure (CPAP). However, new-onset CSA has been described with use of other treatments for OSA, including tracheostomy, maxillofacial surgery, and mandibular advancement device. We present a patient with CSA beginning after endoscopic sinus and nasal surgery for nasal obstruction in the setting of mild OSA. This case highlights the importance of non-PAP mechanisms in the pathogenesis of CompSA.

Investigation of the trophic relations between anaerobic microorganisms from an underground gas repository during methanol utilization

Joint cultivation of the dominant strains of acetogenic, sulfate-reducing and methanogenic microorganisms isolated from water samples of the North Stavropol underground gas storage facility (UGSF) was carried out for revealing their probable trophic relationships. It was shown that acetogenic strains Eubacterium limosum AG12 and Sporomusa sphaeroides AG8-2 growing on methanol could form a considerable pool of hydrogen, which may support development of hydrogenotrophic cultures, the methanogen Methanobacterium formicicum MG134, or the sulfate reducer Desulfovibrio desulfuricans SR12. Growth of this sulfate-reducing strain was not stimulated under joint cultivation with Methanosarcina barkeri MGZ3 on methanol, probably due to its inability to take up low hydrogen concentrations observed during methanosarcina development. The results show that acetogens in the UGSF system are the most important consumers of methanol and hydrogen and after exhaustion of the latter and switching over to methanol utilization they can supply hydrogen to other microorganisms, including methanogens and sulfate reducers. The role of methanosarcina in the UGSF increases as the hydrogen and CO2 reserves are exhausted, and methanogenesis on methanol becomes the main way of its destruction.

Increased Propensity for Central Apnea in Patients with Obstructive Sleep Apnea

There is increasing evidence of increased ventilatory instability in patients with obstructive sleep apnea (OSA), but previous investigations have not studied whether the hypocapnic apneic threshold is altered in this group. To compare the apneic threshold, CO2 reserve, and controller gain between subjects with and without OSA matched for age, sex, and body mass index. Hypocapnia was induced via nasal mechanical ventilation for 3 minutes. Cessation of mechanical ventilation resulted in hypocapnic central hypopnea or apnea depending upon the magnitude of the hypocapnia. The apnea threshold (Pet(CO2)-AT) was defined as the measured Pet(CO2) at which the apnea closest to the last hypopnea occurred. The CO2 reserve was defined as the change in Pet(CO2) between eupneic Pet(CO2) and Pet(CO2)-AT. Controller gain was defined as the ratio of change in Ve between control and hypopnea or apnea to the DeltaPet(CO2). Eleven pairs of subjects were studied. There was no difference in the Pet(CO2)-AT between the two groups. However, the CO2 reserve was smaller in the subjects with OSA (2.2 +/- 0.6 mm Hg) compared with the control subjects (4.5 +/- 1.4 mm Hg; P < 0.001). The controller gain was increased in the subjects with OSA (3.7 +/- 1.3 L/min/mm Hg) compared with the control subjects (1.6 +/- 0.5 L/min/mm Hg; P < 0.001). Controller gain decreased and CO2 reserve increased in seven subjects restudied after using continuous positive airway pressure for 1 month. Ventilatory instability is increased in subjects with OSA and is reversible with the use of continuous positive airway pressure.

Strong Chemoreflex Modulation of Sleep-Breathing: Some Answers but Even More Questions

Strong Chemoreflex Modulation of Sleep-Breathing: Some Answers but Even More Questions

CO2 storage resources, reserves, and reserve growth: Toward a methodology for integrated assessment of the storage capacity of oil and gas reservoirs and saline formations

Abstract Geologically based methodologies to assess the possible volumes of subsurface CO 2 storage must apply clear and uniform definitions of resource and reserve concepts to each assessment unit (AU). Application of the current state of knowledge of geologic, hydrologic, geochemical, and geophysical parameters (contingencies) that control storage volume and injectivity allows definition of the contingent resource (CR) of storage. The parameters known with the greatest certainty are based on observations on known traps (KTs) within the AU that produced oil, gas, and water. The aggregate volume of KTs within an AU defines the most conservation volume of contingent resource. Application of the concept of reserve growth to CR volume provides a logical path for subsequent reevaluation of the total resource as knowledge of CO 2 storage processes increases during implementation of storage projects. Increased knowledge of storage performance over time will probably allow the volume of the contingent resource of storage to grow over time, although negative growth is possible.

The CFZ™ process: A cryogenic method for handling high- CO2 and H2S gas reserves and facilitating geosequestration of CO2 and acid gases

The presence of ever higher contents of CO2 and H2S in natural gas resources presents significant economic challenges for their development and commercialization and for the safe disposal of CO2 and acid gases. They share some of the same challenges faced in capturing CO2 from flue gas. In fact, conventional natural gas treating technologies have been suggested to treat flue gas. CFZ™ is a cryogenic distillation process for the single step removal of CO2 and H2S from natural gas. It is capable of advantageously handling natural gases with a wide range of CO2 and H2S content.

Cryogenic Technology for Processing High-CO2 and -H2S Gas Reserves

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper IPTC 12708, "Controlled Freeze ZoneTM Technology for Enabling Processing of High CO2 and H2S Gas Reserves," by J.A. Valencia, P.S. Northrop, and C.J. Mart, ExxonMobil, originally prepared for the 2008 International Petroleum Technology Conference, Kuala Lumpur, 3-5 December. The paper has not been peer reviewed. Global natural-gas reserves increasingly are reported with significant CO2 and/or H2S content. Concentrations of 20 to 40% are not uncommon. Commercialization of such resources poses significant economic and technical challenges. Approximately one-sixth of ExxonMobil's hydrocarbon resources are categorized as sour gas. With these volumes, ExxonMobil has the industry's largest sour-gas resource base and has more than 50 years experience in managing sour gas. Introduction Monetization of lesser amounts of saleable methane/hydrocarbons while at the same time producing, removing, and disposing of major quantities of acid-gas contaminants calls for technologies that can reduce capital and operating costs. The proper disposal of these contaminants adds to the challenge. Release of CO2 to the environment is not desirable, its geosequestration is a much better alternative, and it would be better yet if it can be turned into a product stream to be sold or used for enhanced-oil-recovery (EOR) purposes. Similarly, alternatives for the disposal of H2S are needed because the conventional conversion to elemental sulfur led to the saturation of many markets. While there has been a recent resurgence in the demand for sulfur, uncertainties about wide-spread demand sustainability encourages the development of alternative H2S-disposal options. The cryogenic technology, developed at ExxonMobil Upstream Research Company, is being advanced to the commercially ready stage in response to these needs. The first choice for many separations in the gas industry are those that take advantage of the simplicity provided by differing relative volatilities and phase behavior and are implemented by flash drums, separators, and distillation and fractionation towers. In the presence of methane, and to yield sales-quality gas, such separations imply very low, cryogenic temperatures of −120°F and lower, depending upon the operating pressure. CO2 solidifies at these low temperatures. The experimental measurement and analysis of CO2-solidification conditions in the presence of methane were pioneered in 1954. Shortly thereafter, a new process was proposed for freezing CO2 from natural gas by flashing a cooled gas stream into solidification conditions and using a cyclonic device to separate the resulting slurry into solid CO2 and liquid methane.

Chronic intermittent hypoxia increases the CO2 reserve in sleeping dogs

We hypothesized that chronic intermittent hypoxia (CIH) would induce a predisposition to apnea in response to induced hypocapnia. To test this, we used pressure support ventilation to quantify the difference in end-tidal partial pressure of CO(2) (Pet(CO(2))) between eupnea and the apneic threshold ("CO(2) reserve") as an index of the propensity for apnea and unstable breathing during sleep, both before and following up to 3-wk exposure to chronic intermittent hypoxia in dogs. CIH consisted of 25 s of Pet(O(2)) = 35-40 Torr followed by 35 s of normoxia, and this pattern was repeated 60 times/h, 7-8 h/day for 3 wk. The CO(2) reserve was determined during non-rapid eye movement sleep in normoxia 14-16 h after the most recent hypoxic exposure. Contrary to our hypothesis, the slope of the ventilatory response to CO(2) below eupnea progressively decreased during CIH (control, 1.36 +/- 0.18; week 2, 0.94 +/- 0.12; week 3, 0.73 +/- 0.05 l.min(-1).Torr(-1), P < 0.05). This resulted in a significant increase in the CO(2) reserve relative to control (P < 0.05) following both 2 and 3 wk of CIH (control, 2.6 +/- 0.6; week 2, 3.7 +/- 0.8; week 3, 4.5 +/- 0.9 Torr). CIH also 1) caused no change in eupneic, air breathing Pa(CO(2)); 2) increased the slope of the ventilatory response to hypercapnia after 2 wk but not after 3 wk compared with control; and 3) had no effect on the ventilatory response to hypoxia. We conclude that 3-wk CIH reduced the sensitivity of the ventilatory response to transient hypocapnia and thereby increased the CO(2) reserve, i.e., the propensity for apnea was reduced.

The apneic threshold during non-REM sleep in dogs: sensitivity of carotid body vs. central chemoreceptors

The relative importance of peripheral vs. central chemoreceptors in causing apnea/unstable breathing during sleep is unresolved. This has never been tested in an unanesthetized preparation with intact carotid bodies. We studied three unanesthetized dogs during normal sleep in a preparation in which intact carotid body chemoreceptors could be reversibly isolated from the systemic circulation and perfused. Apneic thresholds and the CO(2) reserve (end-tidal Pco(2) eupneic - end-tidal Pco(2) apneic threshold) were determined using a pressure support ventilation technique. Dogs were studied when both central and peripheral chemoreceptors sensed transient hypocapnia induced by the pressure support ventilation and again with carotid body isolation such that only the central chemoreceptors sensed the hypocapnia. We observed that the CO(2) reserve was congruent with4.5 Torr when the carotid chemoreceptors sensed the transient hypocapnia but more than doubled (>9 Torr) when only the central chemoreceptors sensed hypocapnia. Furthermore, the expiratory time prolongations observed when only central chemoreceptors were exposed to hypocapnia differed from those obtained when both the central and peripheral chemoreceptors sensed the hypocapnia in that they 1) were substantially shorter for a given reduction in end-tidal Pco(2), 2) showed no stimulus: response relationship with increasing hypocapnia, and 3) often occurred at a time (>45 s) beyond the latency expected for the central chemoreceptors. These findings agree with those previously obtained using an identical pressure support ventilation protocol in carotid body-denervated sleeping dogs (Nakayama H, Smith CA, Rodman JR, Skatrud JB, Dempsey JA. J Appl Physiol 94: 155-164, 2003). We conclude that hypocapnia sensed at the carotid body chemoreceptor is required for the initiation of apnea following a transient ventilatory overshoot in non-rapid eye movement sleep.

Prevention of acute mountain sickness by acetazolamide: as yet an unfinished story

since the 1970s, over 200 studies with acetazolamide have shown it safe and 60–80% effective in acute mountain sickness (AMS). Despite much investigation, our understanding of its action in AMS remains incomplete and more complicated than generally taught. This should come as no surprise

Increased propensity for apnea in response to acute elevations in left atrial pressure during sleep in the dog

Periodic breathing is commonly observed in chronic heart failure (CHF) when pulmonary capillary wedge pressure is abnormally high and there is usually concomitant tachypneic hyperventilation. We hypothesized that acute pulmonary hypertension at pressures encountered in CHF and involving all of the lungs and pulmonary vessels would predispose to apnea/unstable breathing during sleep. We tested this in a chronically instrumented, unanesthetized dog model during non-rapid eye movement (NREM) sleep. Pulmonary hypertension was created by partial occlusion of the left atrium by means of an implanted balloon catheter in the atrial lumen. Raising mean left atrial pressure by 5.7 +/- 1.1 Torr resulted immediately in tachypneic hyperventilation [breathing frequency increased significantly from 13.8 to 19.9 breaths/min; end-tidal P(CO2) (P(ET(CO2))) fell significantly from 38.5 to 35.9 Torr]. This tachypneic hyperventilation was present during wakefulness, NREM sleep, and rapid eye movement sleep. In NREM sleep, this increase in left atrial pressure increased the gain of the ventilatory response to CO2 below eupnea (1.3 to 2.2 l.min(-1).Torr(-1)) and thereby narrowed the CO2 reserve [P(ET(CO2)) (apneic threshold) - P(ET(CO2)) (eupnea)], despite the decreased plant gain resulting from the hyperventilation. We conclude that acute pulmonary hypertension during sleep results in a narrowed CO2 reserve and thus predisposes toward apnea/unstable breathing and may, therefore, contribute to the breathing instability observed in CHF.

Increased propensity for apnea via dopamine-induced carotid body inhibition in sleeping dogs

We determined the effects of specific carotid body chemoreceptor inhibition on the propensity for apnea during sleep. We reduced the responsiveness of the carotid body chemoreceptors using intravenous dopamine infusions during non-rapid eye movement sleep in six dogs. Then we quantified the difference in end-tidal Pco(2) (Pet(CO(2))) between eupnea and the apneic threshold, the "CO(2) reserve," by gradually reducing Pet(CO(2)) transiently with pressure support ventilation at progressively increased tidal volume until apnea occurred. Dopamine infusions decreased steady-state eupneic ventilation by 15 +/- 6%, causing a mean CO(2) retention of 3.9 +/- 1.9 mmHg and a brief period of ventilatory instability. The apneic threshold Pet(CO(2)) rose 5.1 +/- 1.9 Torr; thus the CO(2) reserve was narrowed from -3.9 +/- 0.62 Torr in control to -2.7 +/- 0.78 Torr with dopamine. This decrease in the CO(2) reserve with dopamine resulted solely from the 20.5 +/- 11.3% increase in plant gain; the slope of the ventilatory response to CO(2) below eupnea was unchanged from normal. We conclude that specific carotid chemoreceptor inhibition with dopamine increases the propensity for apnea during sleep by narrowing the CO(2) reserve below eupnea. This narrowing is due solely to an increase in plant gain as the slope of the ventilatory response to CO(2) below eupnea was unchanged from normal control. These findings have implications for the role of chemoreceptor inhibition/stimulation in the genesis of apnea and breathing periodicity during sleep.

The essential role of carotid body chemoreceptors in sleep apnea.

Sleep apnea is attributable, in part, to an unstable ventilatory control system and specifically to a narrowed "CO2 reserve" (i.e., the difference in P(a)CO2 between eupnea and the apneic threshold). Findings from sleeping animal preparations with denervated carotid chemoreceptors or vascularly isolated, perfused carotid chemoreceptors demonstrate the critical importance of peripheral chemoreceptors to the ventilatory responses to dynamic changes in P(a)CO2. Specifically, (i) carotid body denervation prevented the apnea and periodic breathing that normally follow transient ventilatory overshoots; (ii) the CO2 reserve for peripheral chemoreceptors was about one half that for brain chemoreceptors; and (iii) hypocapnia isolated to the carotid chemoreceptors caused hypoventilation that persisted over time despite a concomitant, progressive brain respiratory acidosis. Observations in both humans and animals are cited to demonstrate the marked plasticity of the CO2 reserve and, therefore, the propensity for apneas and periodic breathing, in response to changing background ventilatory stimuli.

Potential Mississippi Oil Recovery and Economic Impact From CO2Miscible Flooding

Maturing of Mississippi oil reservoirs has resulted in a steady decline in crude oil production in the state. To evaluate the potential of enhanced recovery processes, particularly in the use of the state's large CO2 reserves, for arresting this trend, the subject study was performed. A computer data base of over 1315 Mississippi reservoirs was established. All reservoirs were screened for applicability of the carbon dioxide miscible process. With models developed by the National Petroleum Council and DOE, incremental oil that could be produced from the carbon dioxide miscible process was calculated. Under selected economic conditions, carbon dioxide miscible flooding with utilization of carbon dioxide from the state's Norphlet formation (3-7 tcf reserves of high-purity CO2) could produce 120 million barrels of incremental oil in Mississippi. Incremental state revenues as a consequence of this production were calculated to be $45 million of severance taxes, $50 million of corporate income taxes, and $60 million of royalty payments, expressed as present values.

Oil and Gas Developments in Western Canada in 1984

ABSTRACT Petroleum industry activity in western Canada during 1984 rebounded strongly to near-record levels, with the third successive annual increase following the 1981 slump. The year was marked by increased development drilling -- especially in Saskatchewan, further emphasis on in situ heavy oil wells, and a continuation of government incentives. Total number of wells drilled increased by 30% to 8765. Exploratory drilling reversed a three-year decline by rising 45% to 2863 wells. Development drilling reached a record 5902 wells, up 23% over 1983 and 15% over the 1980 record. Exploratory success rate decreased from 62% to 60%, with 1089 oil discoveries and 637 gas discoveries. The development success rate remained at 89%, with 4010 oil completions and 1228 gas completions. Exploratory drilling rose sharply in every area except Manitoba, and development drilling increased in all areas. Average well depths rose marginally in every province but Manitoba. Land sale revenue continued to soar, increasing by 44% to $812 million3, but still below the 1980 peak of $1311 million. Alberta land sale revenue totalled $624 million, Saskatchewan $124 million, British Columbia $62 million and Manitoba $2.5 million. Average price per hectare declined in the two most active areas, Alberta and Saskatchewan, but increased in British Columbia and Manitoba. Alberta exploratory and development drilling reversed a three-year decline and totalled 61% of western Canadian activity. Most of the significant discoveries were made in Devonian carbonates in the Chigwell-Bashaw area, Slave and Kitty areas, Peerless Lake, Rainbow-Shekilie and Dizzy Creek - Steen River areas. A new Lower Cretaceous clastics pool was discovered at Manyberries. Further development and expansion of oil sands projects were announced by Shell Canada in the Peace River area, by Suncor at Fort McMurray and by Esso at Cold Lake. British Columbia activity was centered around last year's Mississippian oil discovery at Desan and another Mississippian gas find at Sikanni. Also, significant CO2 reserves were found by Shell in the Flathead area, just north of the Montana border, and interest increased in west coast offshore potential, despite a 12-year moratorium. Saskatchewan activity expanded sharply for both exploratory and development wells, and was concentrated on the Lower Cretaceous Viking play in the Kindersley-Kerrobert area, Lower Cretaceous heavy oil development at Lloydminster, and a Mississippian play in the Midale-Estevan area. Husky Oil finally announced plans to proceed with a $3.2 billion heavy oil upgrader near Lloydminster. Frontier activity reached record levels due to a continuation of the federal Petroleum Incentives Program and Esso's major development program at Norman Wells. Significant discoveries were made in the Beaufort region, with oil and gas found in a large fault-bounded structure at Amauligak in the Mackenzie Delta offshore, and the largest wet gas discovery onshore at Tuk on the Tuk peninsula. Other Beaufort oil discoveries at Pitsiulak and a Tarsiut delineation well were not as large as expected. Only one of four Arctic Islands wells, namely a stepout well in the Skate oil and gas field off Lougheed Island, was a discovery. The year 1984 marked a return to better times, and all indications point to even greater drilling activity in 1985. RESUME DEVELOPPEMENTS PETROLIER DANS L'OUEST CANADIEN, 1984 Nous regrettons extrement le manque d'une version francaise de cet article assez tot pour la publication. Une version fera imprimee dans le numero prochain (v. 34, no. 1), sur une page a part. 1 Also printed in American Association of Petroleum Geologists Bulletin, v. 69, p. 1491-1500. 4 Money in Canadian dollars Valuable data were obtained from the Canadian Petroleum Association, Canadian Oil Scouts Association, Oilweek and Nickle's Daily Oil Bulletin. We extend appreciation to E. M. Manko, J. Hogg, R. J. Knight and I. A. McIlreath for critical review, to Petro-Canada's Graphics and Design Group for drafting the illustrations, and to Jan Brown and Diana Gillrie for typing the original manuscript. End_Page 410------------------------

Response of North Cowden and Goldsmith Crudes to Carbon Dioxide Slugs Pushed by Nitrogen

Fischer, Daniel D.; Texas A and M U. Bilhartz, Dale; Texas A and M U. Holt, Charles; SPE, Texas A and M U. Johnson, Michelle; SPE, Texas A and M U. Breeze, B.J.; SPE, Texas A and M U. Aud, William W.; SPE, Texas A and M U. Crawford, Paul B.; SPE, Texas A and M U. Texas Petroleum Research Committee Summary Laboratory studies have been conducted to determine oil recovery of the North Cowden-Grayburg and Goldsmith 5,600-ft [1700-m] Clearfork crudes of west Texas when pushed by CO2 at various pressures. Studies were made at reservoir temperatures of 100 and 111 degrees F [38 and 44 degrees C] for the North Cowden and Goldsmith 5,600-ft [1700-m] crudes, respectively. At these temperatures, it was found that the oil recovery ranged from 57 to 99% as the CO2 pressure increased from 700 psi to 1,800 psi [4.8 to 12.4 MPa]. Normally the North Cowden crude gave a higher oil recovery throughout the pressure range. In recognition that CO2, may be in short supply, studies were made of the possibility of using a slug of CO2 pushed by nitrogen. For the slug tests the CO2 slugs ranged from 2.5% to 25% HCPV. All CO2 slugs were pushed by nitrogen. The oil recovery ranged from approximately 60 to 99% over this slug size. The data were obtained in slim-tube equipment ranging from 40 to 100 ft [12 to 30 m] in length. The miscible bank formed between the CO2 and the body of the crude oil was observed to be a clear strawcolored liquid. The analyses of the clear liquid and crude oil conclusively demonstratethat the crude oil undergoes a continuous fractionation process when miscibly displaced by CO2,the need for tong flow tests, andthe inability to interpret most PVT cell data in terms of miscibility. Introduction Most U.S. reservoirs have been substantially depleted by primary production and many of these reservoirs are now in the terminal stages of waterflooding. If abandoned, these oil reservoirs may still leave more oil underground than they have produced. Tertiary oil recovery methods are being developed to recover a large fraction of the original oil in place. One of the enhanced recovery methods receiving consideration is a process whereby a slug of CO2 is injected to achieve miscibility with the crude oil and the CO2 slug is displaced with nitrogen. This method reduces the amount of CO2 required, stretches the CO2 supply, and usually reduces the cost. This would permit a wider application of the CO2 miscible recovery process and, we hope, recover additional oil from our reservoirs. Nitrogen may cost approximately one half as much per thousand cubic feet (Mcf) [28 m 3] as CO2. For many oil reservoirs, 1 Mcf 129 m3] of nitrogen may occupy about three times as much reservoir pore space as the same amount of CO2. Most of the known CO2 reserves are located in Colorado, New Mexico, Utah. and Mississippi. Much of the CO2 target oil is in the Permian Basin of west Texas and New Mexico. A pipeline will be required to transport CO2 to the prospective oil reservoirs. Several companies are moving forward with line construction. The hypothesized CO2 flood potential for west Texas alone has been estimated at 3 to 4 billion bbl [0.48 ⨯ 10(9) to 0.76 ⨯ 10(9) m3] of oil. This will require a lot of CO2. For a very thorough review of CO2 flooding, see Ref. 1, which cites more than 60 references. Whereas CO2 for miscible displacement is in short supply, nitrogen is plentiful. The nitrogen would be obtained from the air by a cryogenic process. In this process, air is compressed and subsequently cooled to approximately –300 degrees F [–184 degrees C]. At this temperature the air liquefies and permits fractionation and purification. The pure nitrogen is taken from the top of the fractionating column and warmed to a gaseous state near atmospheric temperature. The warm gaseous nitrogen is compressed for injection into the oil reservoir. JPT P. 96^