SPE 142851

Quebrache Field: Evaluations to Date of this Natural CO2 Reservoir Heron Gachuz-Muro, SPE, Pemex-Heriot Watt University; Jose L. Sanchez-Bujanos, SPE, Pemex; Israel Castro-Herrera, SPE, UNAM; Jose A. Rodriguez-Pimentel, SPE, Schlumberger

Copyright 2011, Society of Petroleum Engineers This paper was prepared for presentation at the SPE EUROPEC/EAGE Annual Conference and Exhibition held in Vienna, Austria, 23–26 May 2011. This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright.

Abstract In the search for oil and gas during the past century, other gases have been encountered. These gases had little or no economic value and areas known to contain them were avoided during drilling. Deposits of CO2 rich gas (>50 %) are present worldwide but in limited areas – USA, mainly. Few studies of natural CO2 reservoirs are currently available to determine and analyze its accurate exploitation. CO2 concentrations ranging between 71 and 98 % have been discovered in the Northeast of Mexico. Preliminary evaluations (SPE-107445) of the available data for Quebrache field indicated potential gas reserves. Complementary analyses to date have shown that on balance, the Quebrache field offers a significant opportunity for developing Enhanced Oil Recovery (EOR) projects. This new study divides the field into tree important areas. This paper presents: a) recent reservoirs discovered b) estimated reserves for all tree areas with CO2 sources (Central Area, Northern Area and Southern Area), c) efforts made to evaluate its potential d) opportunities to invest in and operate a world-class CO2 reservoir, etc. The Central Area reveals 2 important reservoirs. These reservoirs are relatively continuous and could produce and drain reserves during long period. Original Gas-In-Place (OGIP) volumes are likely conservative because in its calculation it is assumed a gas-water contact (there is contact apparent in the well logs). The Quebrache field would provide strategic value to CO2 injection programs. The CO2 accumulations described in this paper could play a major role in recovering additional oil from fields in the North of Mexico. Thus, CO2 accumulations in the right place and at the right time may become production targets in the future. Introduction Carbon dioxide (CO2) is found both free and combined with other gases, such as petroleum gas and flue gases resulting from combustion. CO2 is marketed either as a solid (dry ice) or as a liquid. Even if the ultimate use of the carbon dioxide requires it to be a gas, it is purchased from the manufacturer as a solid or liquid and sublimed or reduced to a gas at the place of use. Prior to 1970, the main uses of CO2 produced were numerous: refrigeration, heat and cold treatement, laboratory uses, carbonating beverages, fire extinguishing, etc. The volume for these purposes was small. During the 1970’s, however, a new use for CO2 emerged. Injection of this gas into mature oil fields could mobilize oil that has been left behind by primary or secondary recovery techniques. This new application substantially increased demand for carbon dioxide. Mainly, two main factors have made carbon dioxide an attractive resource target in some specific areas, a) it has shown that in Enhanced Oil Recovery (EOR) processes can increase oil production and b) the rise of the price of oil has made attractive its exploitation. Part of the economic feasibility of these EOR methods is a source of carbon dioxide which can be transported and injected at a reasonable cost. Sources of CO2 are quite diverse but there are three primary options: a) Supplies from natural reservoirs, b) Anthropogenic sources or c) Recicled CO2.

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Natural sources of CO2 occur, as gaseous accumulations of CO2, CO2 mixed with natural gas, CO2 dissolved in formation water or coexisting with oil fields as dissolved gas and gas cap.These accumulations have been studied in the United States, Hungary, Australia, Romania, Turkey, etc. Natural accumulations take place in a number of different types of sedimentary rocks, principally limestones, dolomites and sandstones and with a variety of seals and a range of trap types, reservoir depths and CO2 bearing phases. Despite the amount of information available from these sites, in many natural CO2 reservoirs the source of the CO2 and basin scale processes that act on them are poorly understood. This is partially due to the multiple origins of CO2 in natural gases. These include methanogenesis, oil field biodegradation, kerogen decarboxyilation, hydrocarbon oxidation, decarbonation of marine carbonates, degassing of magmatic bodies, etc. In Quebrache, it seems reasonable to think that the origin of the CO2 is closely related to an inorganic origin. The gases of Quebrache were generated from the primary cracking of kerogen, corresponding to an open system without any evidence of secondary cracking. In many cases (concerning to the oil industry), when CO2 is found as a natural source, it is an unwelcomed dilutent and corrosive agent in hydrocarbon natural gases. These uncommon instances were traditionally classified as failure, i.e., the gases had little or no economic value and areas known to contain them were commonly avoided during further drilling. EOR projects using CO2 have risen dramatically in recent years. More than 100 CO2 projects were reported in 2010, figure 1. Numerous other developments were announced and planned. Nevertheless, the supply is of great concern to the oil companies. In order for this technology to be safely implemented the long term consequence of injecting CO2 into the reservoirs must be quantifed.

Figure 1.-USA EOR Projects under Gases, (source OGJ-2010).

The objective of the present paper is to assess the CO2 reserves of Quebrache field in 660 km2 under traditional methods published. The results of these studies corroborate its potential EOR application. It is therefore important to point out that Quebrache field, in the right place and at the right time, may become production target in the future. Historical Development Beginning in 1901, prolific oil areas were discovered in Northeastern Mexico. Drilling in the southwestern portion of Tampico

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found mainly high-purity CO2 or variable gas mixtures with high concentrations of CO2. Only sporadic exploration efforts continued in the area in the search for oil and gas. CO2 had little or no economic value (absence of a CO2 market) and areas known to contain it were avoided during drilling. Quebrache field was discovered in 1915. The producing formations are rocks of Cretaceous and Tertiary ages. It is still largely undeveloped and it extends over an area of 2,500 km2. The discovery well encountered high-purity CO2 (90 %) with minor amounts of nitrogen and the rest being hydrocarbon gases. The well remained closed few years. Interest in CO2 development remained low until exploitation studies, in the late 1990’s, indicated that operations of gas lift could lead to the successful recovery of additional oil. Production began in 1997 and has continued at a rate of approximately of 2.2 MMscfd since then. Interest in this field increased because it was the closest and best source of CO2 to develop an EOR program in a mature oil field. Two years ago, a project study group was formed to cover all aspects of a possible development. Part of this revision divided the field into tree important areas: a) Central Area b) Northern Area and c) Southern Area (figure 2). The Central Area is more than 85 % pure whereas the Northern Area and Southern Area contain between 60-92 % mol. Neither structure has been fully developed. Currently, 17-20 wells provide about 12 MMscfd of carbon dioxide. This volume is transported via pipeline to a nearby heavy oil field for gas lift operations.

Figure 2.-CO2 accumulations in Quebrache Field (660 km2 was evaluated).

Recent Studies This section of the paper is divided by areas of importance for its reading. Each area was reviewed in detail with the available data.

• The Central Area The review of various electric logs, several well tests and analyses of recovered gas led the existence of natural CO2 into 231 km2. Numerous structures in the zone have been defined as CO2 targets. Two of these structures have been tested. Although tests have proven the new wells are capable of producing at rates of more than 4 MMscfd, production has been limited by pipeline dimensions. The concentrations exceed 85 % purity. A preliminary evaluation of the data for this area indicated recovery potentials (SPE-107445). We have divided this section informally into upper and lower members because of rock quality. The secondary formation (upper member) does not have enough information. The lower member is the biggest of these reservoirs and contains estimated reserves up to 1 Tscf. We focused on this unit which provided complementary information. A well was exploited and operated for industrial use; however, the main information is unknown. For this reason, we decided to exclude it from our evaluations. These formations are occurring at depths of 900-1100 m and consisting of a 9-45 m thick, upper member, and 13-40 m thick, lower member. The wells also produce minimal amounts of formation water and condensate (1 bl/MMscf and 5-10 bls/MMscf, respectively). One well with sufficient information was selected representing a natural behavior in this area. We used the method stablished by Cinco-Ley to evaluate its latent volume. The adjusted model was “Infinitely Acting Reservoir”, figure 3. A second case then, figure 4, was run where a closed system was simulated. This allowed obtaining a proven volume, 329.84 Bscf. The volume confirmed the presence of relatively continuous formations where the CO2 could be produced and drained during long period.

MEXICO

U.S.A.

GUATEMALA

GOLFO DE

MEXICO

OCEANOPACIFICO

TOPILA

QUEBRACHE

TAMPICO

MEXICO

U.S.A.

GUATEMALA

GOLFO DE

MEXICO

OCEANOPACIFICO

MEXICO

U.S.A.

GUATEMALA

GOLFO DE

MEXICO

OCEANOPACIFICO

TAMPICO

Gas

Oil

Total Area (660 Km2)

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Figure 3.-Pressure and production data representing a large radius of investigation.

Figure 4.-Adjusted data under a closed system.

The reservoir drive is presumed to be a depletion drive, although an active acuifer is possible (there is a contact apparent in the well logs). Recovery efficiency is assumed to be 70 % because the formations are relatively continuous units. The limited production tests have revealed an infinitely acting reservoir. One of the advantages of the horizontal well is to achieve large reservoirs contact area. The effects of long horizontal wells on production rate also were scrutinized. The search provided the selection of a reasonable horizontal well length (200-300 m). Figure 5 illustrates the variation of gas production rate with well length for different chokes. The chokes used are diverse because of the low reservoir permeability. After a certain point, it is seen that the curves stayed invariable which showed that a rise in the horizontal well length does not yield a corresponding growth in the production.

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Figure 5.- Effects of chokes on production rate (kv= 3.5 md, kh= 8.2 md)

It is clear that the permeability has a high effect in production rate indicating an important parameter in optimum well construction. Vertical permeability is one of the key parameters which could determine the productivity of this area.

• The Northern Area Seismic data (2D seismic) was essential for the identification of new zones. The quality of available data was sufficient to map structure but insufficient to evaluate correctly seismic stratigraphy. The Northern Area varies significantly throughout the zone from 60 to 92 % CO2. To date, a total of 10 wells are being exploited. The largest active production occurs at this mature area. The oldest wells are erratics and not representatives of the reservoir as a whole. Nevertheless, this group of wells provided log data, stratiraphic column, drilling depths, etc. The completion techniques commonly used for these wells were openhole completion methods. This method represented to be troublesome due to the difficulties for monitoring data besides did not to prove acceptable for workover operations. Net thickness ranges from 9-45 m with an estimated pressure of 847 psi. Reservoir porosity is ranging from 5 to 16 % but the permeability is very low. Well tests showed effective permeabilities in the range of 1 md. Production testing exhibited that there are no significant volumes of formation water. There is no sufficient pressure to produce except for a very short time. To the northwest of this zone are the active heavy oil fields. Analyses yielded an estimated 0.54 Tscf of estimated reserves. Possible addition to these volumes described above would be in the Southern Area.

• The Southern Area Further analyses delineated a new area with volume of CO2. This discovery includes oil fields where CO2 is found as a gas mixed (50 % carbon dioxide) and H2S is present in amounts that require treatment. Conservately, we assume that this zone is potential. In spite of uncertainties involved, the findings of various revisions identify acceptable evidence that a substantial quantity of gas could be exploited. The area was formed with isopac maps and up to date, 33 km2 as probable area is being borne in mind. Neither volumen has been considered but its evaluation is being contemplated. It is probable that production may be developed from an area as great as the Northern Area.

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Remarks

The primary purpose of this paper was to locate all latent CO2 natural sources within Quebrache Field that could be used to EOR processes.

Quebrache field could provide a strategic source of supply that is proximate in location to significant current and

possible CO2 floods in the North of Mexico. The close proximity of oil fields could insure economic success of the EOR-CO2 projects.

New areas would offer an opportunity in mature fields through EOR methods. Additionally, oil fields with low

recovery factors could be included in secondary recovery processes. CO2 demand in Mexico is expected to stay strong and grow with project expansions and new floods.

Optimization of horizontal well length and well spacing can be critical to the economics of the exploitation of gas

resources for this Region.

Although this paper deals primarily with carbon dioxide gas production in the Northeastern Mexico, the occurrence of the gas at numerous localities and conceivably in large volumes in Mexico must not be overlooked. Mature oil fields could become more competitive with this locally produced gas.

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