Differences between ISC and Air Injection

Transcription

1 Paper No 5 Air Injection: An Alternative IOR Technique to Water Flood Author : Sidhartha Sur, General Manager Heavy Oil, IRS, ONGC, Ahmedabad 38 5 Tel No (O) : , (M) : sur_sidhartha@ongc.co.in Abstract : In-situ combustion (ISC) is an air injection process but all air injection processes can not be classified as in-situ combustion process. In-situ combustion involves injection of air and is an immiscible oxidation process accompanied by high temperature (5-6 C). Primarily it aims at heavy oil. Whereas air injection in light oil can be immiscible or miscible oxidation process accompanied by lower temperature (3-35 C). Miscible or immiscible, it depends on the reservoir setting, depth, pressure and temperature. ONGC has already established in-situ combustion technology in increasing recovery from the heavy oil fields. Commercial ISC in Balol and Santhal fields are already on stream since Potential of the process in both the fields has been manifested in terms of arresting aquifer encroachment and increasing production and recovery. Current recovery from the sectors wherein ISC process is operating for about a decade (phase-i) in Balol and Santhal fields is about 45 % and 38 % respectively compared to the envisaged primary recovery of 13-17%. The setting up of air injection laboratory at IRS in April 26 is a major milestone in the area of widening the domain of application of the process to encompass medium and light oil fields. Time has come to think about light oil fields wherein water injection is a challenge due to poor petrophysical properties or due to nonavailability of required quantity of HC gas or desired water. In a low porosity system, chemical flooding will also be beset with injection of required quantity of desired chemicals. Answer to What Next? is air injection process. Air, unlike other EOR agents, is free and abundant.

2 Air injection holds promise in adding reserves and is also bestowed with uncertainty. To address the challenges and uncertainties, laboratory understanding and field experience have played a major role in the past and will continue to do so in future as well. In 26-7, laboratory investigations have been carried out for several light oil fields of ONGC wherein water injection is a challenge. Results are positive but at the same time bring out to the forefront some more issues that need to be addressed before field testing. Keywords : In-situ combustion, Air Injection, Water Injection, Tight oil reservoirs, Water Injection, Recovery Factor Introduction : In-situ combustion and air injection both employs air. However they are different as the first technique is employed in heavy oils (immiscible) accompanied by high temperature (5-6 C) and second in light oil (miscible or immiscible) accompanied with low temperature (3-35 C). With the successful results of Balol & Santhal commercial in-situ combustion projects of ONGC, India, in-situ combustion (ISC) is back in focus in heavy oil exploitation in unconsolidated clastic reservoirs. Reservoir rocks with low porosity and permeability are not uncommon in the oil industry. These types of reservoirs pose a unique problem in terms of oil production and recovery enhancement. Although water injection is the most common secondary recovery technique employed to recover the second crop of oil, it has encountered challenges in low porosity system. Either availability of required quantity of water is in doubt or injecting the required quantity of desired water is a challenge due to low injectivity. In such a situation oil recovery is affected and results in lower recovery. Gas injection is an ideal technique in such a situation and with gravity on its side it is the preferred choice over all other techniques. Commitment of hydrocarbon gas to downstream markets has put tremendous pressure in the availability of requisite quantity of hydrocarbon gas. Choice is left to non hydrocarbon gases like Nitrogen (N 2 ) and Carbon dioxide 2

3 (CO 2 ). Separation of Nitrogen from air is an expensive process and in the absence of natural deposits of CO 2 separation of CO 2 from flue gases are equally cost intensive. Therefore the question of What Next? gives rise to Air Injection. Air is available in plenty and at no cost and can be an effective tool in enhancing production and recovery. Differences between ISC and Air Injection Both the techniques are based on air. An immiscible air injection process in heavy oil accompanied by high temperature oxidation (HTO) is termed as ISC. A miscible or an immiscible air injection process in light oil accompanied by low temperature (LTO) is termed as air injection. The term High Pressure Air Injection (HPAI) is being used in the industry for air injection process in light oil. It is more to do with administrative reason than technical. In this paper it will be referred to as air injection process for the case of simplicity. Fig-1 The Fig-1 explains the difference of oxidation of heavy oil and light oil. In the case of heavy oil, there exists a negative temperature gradient. The first peak is the region of distillation, dehydrogenation & coke deposition during pyrolisis and the second high temperature peak is that of oxidation of the fuel resulting in the formation of carbon dioxide & water maintaining stoichiometric balance. It is extremely important that in ISC projects, required supply of air is maintained and initiate the process artificially rather than relying on spontaneous combustion to 3

4 ensure bond scisson reactions of the high temperature oxidation peak. The oil mobilization benefit of ISC is then derived. In the light oil, at around 15 C, resins & aromatics are consumed and bond scisson reactions occur at around 3 C. Therefore the so called low temperature region of heavy oil is the zone of bond scisson reaction in light oil. Oxidation in light oil has its advantage than in heavy oil as there is no region of negative temperature gradient that the reactions have to proceed to enter into the bond scisson reaction zone. In fact till date no air injection project in light oil has failed. ISC in Heavy Oil In-situ combustion should be viewed as a displacement technique and not just a mobility enhancement tool for improving recovery. ISC has been commercialized in two phases in Balol & Santhal fields viz Phase-I &II. Phase-I initiated in mid is already ten years on stream and has seen uninterrupted air injection during the period. In this paper, therefore, performance of Phase-I has been reflected. The ISC projects of Balol & Santhal have brought the process back into focus. The strength of the process is manifested in terms of : - Arrest of edge water encroachment - Increase in oil production & recovery The figures 2&3 demonstrate that with continued air injection in the crestal part of the fields and maintaining voidage compensation reasonably close to unity, edge water encroachment could be arrested. Any imbalance in voidage compensation as seen by the last scenario in either fields results in making the aquifer active. Recovery achieved from the phase-i areas of Balol & Santhal is currently of the order of 45 % & 38% respectively (Fig 4&5). This is significantly higher than the envisaged primary recovery of 13 & 17% respectively. 4

5 stoppage Fig-2 Fig-3 Balol Phase-I Santhal : Phase-I Recovery %, No of Producers Recovery Year R e co ve ry % / Pro ducers A p r-7 9 A p r-8 1 A p r-8 3 A p r-8 5 A p r-8 7 A p r-8 9 A p r-9 1 A p r-9 3 A p r-9 5 A p r-9 7 A p r-9 9 A p r- 1 A p r- 3 A p r- 5 A p r- 7 Fig-4 Fig-5 Why to consider air injection? Adding reserves to the portfolio is the order of the day. In view of difficulty in discovering easy to find oil, efforts are on hand to increase recovery from existing fields. Tight oil reservoirs are no exception. Development of low porosity (< 15%) & permeability (< 2 md) has posed unique challenges. Availability of required quantity of water is becoming a problem in many areas, due to the restriction of offtake from ground water sources or semi-arid/arid areas. Inspite of the availability, injecting required quantity of desired water has been a challenge owing to low injectivity. Oil production & recovery gets adversely affected. Gas injection is an ideal technique in such a situation and with gravity on its side it is the preferred choice over all other techniques. Commitment of hydrocarbon gas to downstream markets has put tremendous pressure in the availability of requisite quantity of hydrocarbon gas. Choice is left to non hydrocarbon gases like Nitrogen (N 2 ) and Carbon dioxide (CO 2 ). Separation of Nitrogen from air is an expensive process and in the absence of natural deposits of CO 2, separation of 5

6 CO 2 from flue gas is equally cost intensive. Therefore the question of What Next? gives rise to Air Injection. Air is available in plenty and at no cost and can be an effective tool in enhancing production and recovery. Air injection has a high potential in enhancing recovery due to high displacement efficiency. This has been demonstrated in laboratory evaluations discussed in subsequent section. Air injection process is better understood now than it was about 2 years back. With the arrival of better production procedures & equipment and drilling technologies, air injection process can be more attractive than it was in the past. Laboratory Evaluations Laboratory evaluations have been carried at IRS, ONGC, Ahmedabad on the new air injection set-up (Fig-7) commissioned in association with University of Calgary, Canada. Fig-6 Air injection experiments targeted high temperature (> 7 C) clastic reservoirs with poor porosity & permeability. The fields A, B, C, D are located in the western part of the country in the State of Gujarat. Some characteristics of the fields are given below : 6

7 zone-5 13:29:48 13:48: 14:24:23 zone-6 14:42:34 15:18:56 15:37:7 zone-7 Time, hrs 15::45 15:55:18 16:13:29 16:31:39 zone-8 16:49:5 17:8:1 18:2:32 18:2:42 18:38:52 2:9:4 2:27:49 2:45:59 21:22:19 21:4:28 zone-12 21:4:9 Under water injection 19:51:3 Remarks 19:33:21 zone-11 Field-D 19:15:12 zone-1 18:57:2 zone-9 17:44:21 Flux increased to 5% 17:26:11 Field-B 14:6:11 Res Temp, C 13:11:37 API 12:53:26 zone-4 12:35:14 Permeability, md 12:17:3 Porosity,% 11:58:51 Current Pr,ksc 11:4:4 Initial Pr,ksc zone-3 11:22:28 Field zone-2 11:4: :51:33 1:46:5 35 9:33:23 1:27: zone-1 9:15:13 1:9: :57:3 15 8:38: TIME A temp, deg C T EM P, d eg C 1::2 1:16:7 1:32:12 1:48:17 11:4:21 11:2:26 11:36:32 11:52:37 12:8:43 12:24:49 12:4:55 12:57:2 1:13:9 1:29:15 1:45:22 2:1:28 2:17:34 2:33:41 2:49:47 3:5:53 3:21:59 3:38:5 3:54:12 4:1:18 4:26:24 4:42:3 4:58:35 5:14:41 5:3:46 5:46:5 6:2:55 6:19: 6:35:4 6:51:8 7:7:13 7:23:17 7:39:22 7:55:26 8:11: :42: :19:17 9:52:5 5 22:56:5 9:29: :32:53 97 Zone 7 Core Zone 8 9:6: :9:4 8:43:13 B 21:46:28 8:2:1 1 21:23:16 7:56: ::4 7:33:36 7 2:36: :13: :5: :1:24 19:4: 19:27:13 Zone 6 Core 6:47:12 Zone 7 6:23:59 18:4:48 C 6::47 18:17:35 78 Field-C 5:37:34 Zone 5 Core 5:14:21 Zone 12 Core Zone 6 4:51: :54:22 4:27: :7:55 17:31:8 4:4: :44: :21: :58:14 D 3:41:27 15:35:1 All the fields are having low to moderate dips. The tube runs used native core Zone 5 3:18:13 14:48:33 Time 2:54:58 14:25:2 15:11:47 Zone 4 Core 2:31:44 14:2:6 Field-A 2:8:29 Zone 11 Core Zone 4 Zone 12 1:45:14 13:38:52 material / synthetic sand and oil. Porosity of the fields could not be simulated as 1:21:59 13:15:38 Zone 3 Core 12:58:44 Zone 1 Core Zone 3 Zone 11 12:35:29 Tiimee (Hrs) NAWAG - Temperature profile-exp # 4 12:29:1 12:52:24 12:12:14 sand pack using crushed core material is used in the experimental set-up of 12:5:56 11:48: m long and 1 mm in diameter. Oil saturation of 5-6% was maintained Zone 2 Core Zone 9 Core Zone 2 Zone 1 11:42:42 through restored state technique. Date:7/12/26 11:19:28 Tube Run Results 7 6 1:16:2 11:25:45 5 1:32:59 1:56:13 11:2:32 4 9:46:31 1:9:45 9:52:49 1:39:17 3 9:29:36 9:23:17 Zone 1 Core 9:6:23 2 9::4 Zone 8 Core Zone 1 Zone 9 8:43: :36:51 8:19:58 Fig-7 The temperature profiles of tube runs (Fig 7) carried out indicate peak temperatures of 4-5 C, 3-4 C, 4 C & 5 C for Fields A, B C & D respectively. Stable front could be propagated along the tube with low oxygen in 7 the flue gas. Experience with tube runs of field D was different than with other Temperature, deg C Temp (Deg C)

8 fields. A high temperature front could not be created / initiated in several attempts using native oil, core and air. A joint run was carried out with in-situ combustion research group of University of Calgary at IRS, Ahmedabad. Steam was injected prior to injecting air. A steam plateau of 3 C can be seen in the temperature profile of Field D. In the presence of steam a very stable oxidation front could be created, sustained and propagated along the length of the tube. In all the tube runs, post burn analysis of the swept zone indicated burnt sand devoid of oil & water. Average 1D recovery of 9% was achieved in all the runs. The results of the tube run have been summarized below. Field Flue Gas Composition, CO 2 / O 2, % Air Requirement, Nm3/m3 O 2 Utilisation Efficiency, % A 2 / B 1 / C 17 / D 13 / Conclusions : 1. The low porosity & permeability light oil fields that has been investigated offers an opportunity for application of air injection to add reserves 2. Laboratory investigations indicate that stable oxidation front could be created, sustained and propagated in light oil in all the four cases. 3. Different oils demonstrate varying oxidation characteristics 4. Air injection coupled with smaller spacing between injector & producers can be an alternative to water injection in low permeability reservoirs 8

9 Acknowledgement : The author wish to express his sincere appreciation to Oil and Natural Gas Corporation Limited (ONGC) for permission to publish this paper and to acknowledge the outstanding efforts demonstrated by the personnel of Thermal Laboratory of IRS, ONGC for successfully commissioning the state of the art air injection set-up and carryout air injection investigations in light oil for the first time in this part of the world. The interpretations and conclusions expressed in this paper are solely those of the author and not necessarily those of the organization. References : 1. Unpublished reports of IRS, ONGC on viability of air injection in light oil fields of ONGC 2. Communication with Dr R G Moore, Professor, University of Calgary, Dr S A (Raj) Mehta, Professor, University of Calgary and personnel of in-situ combustion research group of University of Calgary. 9