Oil industry efforts in the Valencia Trough off the Mediterranean coast of Spain have yielded several historical results. Not least among them is the production of 257 million barrels of oil, with no known impact on the marine environment, fisheries and Spain's vital tourism industry. Before the first North Sea oil came on stream in 1975, Shell was already producing oil from the Amposta field through two small platforms built in Spain.
The Casablanca oil field, discovered in 1975 and located on the Mediterranean shelf edge, has also been greatly significant in the world’s offshore oil industry activity, besides being by far the biggest oil field in Spain.
Amposta and “The Well That Discovered Itself”
In 1970, a consortium led by Shell discovered the Amposta oil field with the second exploration well drilled in the Spanish Mediterranean and the first oil discovery in the Valencia Trough. The successful well drilled a large and obvious tilted horst-like feature identified by 2-D seismic and confirmed the presence of a significant unconformity at the base of the Cenozoic section, before penetrating heavily fractured and karstified Mesozoic carbonates where circulation was lost immediately. Shell pumped seawater and mud into the well to cure the circulation loss, then decided to abandon it. The mud pumps broke down while cementing the well and, unexpectedly and luckily, the well started to flow heavy oil (17 degrees API) from the carbonates – thus revealing a prolific petroleum system and opening a promising future for exploration in the basin.
Soon thereafter, Shell made a second but marginal oil discovery in the area which, together with Spain’s favorable petroleum legislation at that time, encouraged other international petroleum companies to acquire exploration acreage. The shallow waters of the Mediterranean offshore were practically fully covered by exploration permits. Other discoveries were made, all littered about the western continental shelf, but with lighter oils, such as Tarraco’s 36-degree API oil tested by Shell in 1973, culminating with the discovery of 33-degree API oil from Casablanca in September 1975.
‘The Beginning of a Beautiful Friendship’
The Casablanca story really begins in July 1971 with the application by Ranger Oil of Canada for five hydrocarbon exploration permits named Casablanca, Alfaques, San Carlos, Alcanar and Peñiscola, covering a narrow north-south strip of acreage (70 kilometers long by 10 kilometers wide), some 50 kilometers east of the Amposta field. All permits were named after Mediterranean seaside cities except Casablanca, which was a restaurant on the coast where the geologists from Ranger Oil used to have lunch during their field work, recalling fond memories there. In turn, the restaurant was named after the 1942 movie “Casablanca,” the story of political and romantic espionage starring Humphrey Bogart and Ingrid Bergman that takes place in the Moroccan coastal city. The oilfield’s name “Casablanca” resulted in some funny misunderstandings – such as the American driller who came to believe he was working in the Moroccan offshore!
Three quarters of the acreage of the five permits were in water depths exceeding 200 meters, which was then considered the technological limit for field development. However, after the high level of interest emerged in the region, it was not strange that other companies submitted competitive bids to the Ranger’s application. It took two years of deliberation by the Spanish government, further complicated by Shell’s oil discovery in basal Miocene carbonates at the Tarraco structure in April 1973, adjacent to the Casablanca permit.
Finally, in November 1973, the five exploration permits were granted 100 percent to Canadian Northwest Land, a newcomer to Spain with limited financial and technical capacity. CNWL had recently acquired a stake in a risky exploration venture in the Arctic islands, which impressed the Spanish authorities, even though there did not appear to be any prospect of a financial return in the near future.
CNWL had committed to shoot seismic in the five permits during the first two years and, in case of pursuing ahead, to drill one exploration well in each permit during the third and fourth years. Should a commercial discovery be made, a 40-percent carried interest through exploration was also offered by CNWL to a state-owned entity. The late Jim Kirker (he passed away in 2017 at the age of 90), by then the exploration manager in CNWL – whose memoires and oral communications on the Casablanca story are fully acknowledged here – was an enthusiastic geologist who will be remembered for his passion for oil exploration and his important role in the discovery of the Casablanca field. Kirker was deeply involved in this venture, persuading CNWL management to explore offshore Europe, and particularly showing a high confidence in the hydrocarbon potential of the Mediterranean.
Following the awarding, CNWL acquired in 1974 a regional 2-D seismic survey, with which several large structures were identified. In 1975, CNWL successfully farmed out a percentage in all permits to Denison Mines (25 percent) and Chevron (25 percent), who was then appointed operator. Subsequently, the Spanish company Cepsa (12.5 percent) and Pacific Petroleums (12.5 percent) joined the group.
Oil Discovered by a Paleontologist
The first exploration well was drilled by Chevron in July 1975 within the San Carlos permit. It sought to test one of the largest structures, but the well revealed a thick Miocene volcanic section lying on Triassic clastics, the primary objective Mesozoic carbonates being absent. The well had no shows and was plugged and abandoned as a dry hole in August of 1975.
The semisubmersible drilling rig then moved to the next location to test the Casablanca structure, 10 kilometers northeast of Shell’s Tarraco light oil discovery, on the same structurally high trend, which extended further eastward into deeper waters. The Casablanca-1 well, located at the edge of the continental platform in a water depth of 133 meters, was spudded on Aug. 13, 1975. The well encountered the anticipated Mesozoic carbonates uncomformably beneath the Cenozoic section and close to the prognosis depth. Several drilling breaks occurred in the Mesozoic section, but no oil nor gas shows were noted nor loss of circulation while drilling down to total depth at 3,173 meters into the Triassic red beds. Wire-line logs were run, but the petrophysical evaluation did not show a clear interpretation. Fortunately, José Ramírez del Pozo, a prestigious Spanish micro-paleontologist, noted oil staining while washing and crushing some Jurassic carbonate samples for age dating, and immediately alerted the Chevron geologists. These oil shows encouraged testing of the well, in spite of the ambiguous petrophysical results. Testing of the Jurassic carbonates began on Sept. 21, 1975. A total of six drillstem tests were run and the recovered oil averaged 33-degrees API, low-sulphur, with individual flow rates as high as 10,670 barrels of oil per day. Testing confirmed an oil column of up to 200 meters.
The good news of the Casablanca discovery was not released by the Spanish media due to the agony inflicted on the country by Francisco Franco, the head of state who had ruled Spain with an authoritarian fist since 1939. Two months after the Casablanca discovery, on Nov. 20, 1975, Franco died, generating worry as to which direction Spanish politics would take under the regime of his successor, King Juan Carlos I. Soon after his accession, Juan Carlos introduced reforms to dismantle the former regime and begin the Spanish transition to democracy.
The Casablanca discovery then made the Spanish Mediterranean offshore even more attractive, creating a rush to acquire exploration permits, including acreage in water depths greater than 200 meters, immediately east of the Casablanca discovery. Most exploration and drilling activity in Spain at that time was concentrated in this basin and the historical peak drilling activity was reached in 1976, when a total of 19 wells were spudded.
In 1976, after the establishment of the Casablanca field’s commerciality, Eniepsa (at that time the Spanish state-owned oil company that years later became Repsol) was designated as a participant with a 40-percent interest in the five exploration permits, including Casablanca, reducing the other partners’ percentage proportionally.
A Two-Phase Production History
During 1976, one appraisal and three step-out wells were drilled aimed at confirming and delineating the field’s extension. In July 1977, Casablanca-1A, a sidetrack of the discovery well, was placed on an extended production test. A semisubmersible drilling rig was used as the production platform with the oil produced to a moored storage tanker. The test ended in June 1978 after recovering 1.5 million barrels of oil, which allowed the reservoir’s performance to be assessed.
The Casablanca exploitation concession was granted in December 1978 for 30 years, plus two additional 10-year extensions. The concession award also approved a two-phase development plan. The first phase consisted of early production from two subsea completions, producing to a semisubmersible production platform and a moored tanker. The second phase consisted of a fixed production platform with a permanent drilling rig, with production to be pumped to a shore terminal through a pipeline.
However, the Casablanca field was proved by drilling to cover portions of two contiguous concessions: Casablanca and Montanazo D, with most of the structure being located in the Casablanca area. The concessionaires in the Montanazo D block were the same as in Casablanca plus Amoco. After long technical discussions without reaching an agreement, a unit operating agreement was finally settled by writing it on a napkin at the end of an executive lunch held in November 1979. This was approved by the government in June 1980. In the agreement it was established that the pooled Casablanca oil field reserves were 77.8 percent in Casablanca and 22.2 percent in the Montanazo D area, and a Casablanca-Montanazo D Unit area was defined. It also established a unitized reservoir volume and an agreed original oil-water contact at 2,733 meters subsea based on water saturations from logs.
Production from the first phase started at approximately 10,000 barrels of oil per day in December 1979 by a semisubmersible platform built in Spain for that purpose. This platform was owned by the Casablanca group until 1985 when it was sold for $22 million. Production came from the new subsea well Casablanca-6 and the existing Casablanca-1A well, which was recompleted and placed on production in March 1980. In October 1980, export started through a 12-inch, 47-kilometer subsea pipeline to the Tarragona refinery. This early production provided a favorable cash flow during construction of the fixed platform.
In July 1981, the fixed platform jacket was set in 161-meter water depth – a complex and sensitive operation. The installation of topsides for fluid processing, drilling, power generation and accommodation facilities was completed by year-end with a design life at the time of 25 years. The jacket, as well as most of the topsides, was fabricated in Spain, and at that time was considered an outstanding technological landmark. By early 1982, the existing wells were tied via submarine flowlines to the fixed platform and deviated wells were drilled from the platform rig and placed on production.
Two 3-D seismic surveys totaling 132 square kilometers were acquired in 1982 and 1984, among the pioneering offshore 3-D surveys worldwide. The surveys allowed the identification of two structural culminations, which could not be drained by the existing wells and were not reachable from the fixed platform. Therefore, new subsea wells were drilled in 1984 and tied-back through umbilical’s and pipelines to the platform.
In 1982, once the facilities were fully operational, Eniepsa (now Repsol) assumed the operatorship of the Casablanca platform and the drilling of new development wells, including some extended reach wells with horizontal departure in excess of 4 kilometers, a European record by then. The peak production was achieved in 1983 with a daily offtake of 44,000 bopd in a $30 per barrel scenario.
Excepting sidetracks, a total of 18 wells have been drilled in the Casablanca field. There is no Casablanca-13 well, as it was considered an unlucky number. It was skipped and numbering went straight to Casablanca-14, a vertical well drilled in 1984 aimed to drain the northern attic located in a water depth of 500 meters. The well encountered the reservoir 140 meters lower than prognosis, resulting in a dry hole that was then plugged and abandoned. But, what was even worse, taking for granted that the well would be an oil producer, a prototype of subsea tree was specially designed and built for such water depth, by then an industry-wide deep offshore production challenge, but it was not installed.
A Complex but Prolific Reservoir
The Casablanca field is a southwest-northeast trending feature, 11 kilometers long by 1 to 1.5 kilometers wide, with a surface area of nearly 13 square kilometers. The structure is composed of three culminations, the shallowest at 2,465 meters subsea, with a maximum vertical closure of nearly 270 meters. The field produces from fractured, brecciated and highly heterogeneous carbonates, ranging in age from Cretaceous in the northern part of the field to Jurassic in the south. These carbonates had been uplifted, fractured, subjected to a long period of subaerial exposure, karstified prior to being down-dropped and buried by shales, conforming a paleo-geomorphic trap. The primary matrix porosity averages 3 percent, but the occurrence of vuggy porosity combined with an open fracture network provide the means for the reservoir to sustain well flow rates in excess of 10,000 barrels of oil per day. The fact that more than 25 million barrels of oil have been produced already from a single well demonstrates how a seemingly poor-quality reservoir rock was enhanced and transformed by an extensive fracture network and karstification.
Reservoir pressure is supported by a strong regional saline aquifer (some 40,000 parts per million ppm sodium chloride). Reservoir temperature is high, close to 150 degrees Celsius. There is no natural gas cap and the gas-oil ratio is low. Produced gas is used for generation of electricity for the platform’s consumption, and the spare gas is flared.
Production was slowly declining until early 1991, when the drilling of Casablanca-17 revived the field´s performance. Production was by natural flow until 2003 when wells began to be completed with electro semisubmersible pumps. Then water production increased, requiring increased water handling capacity at the platform and the conversion of former producing wells into water disposals. Different new technologies have been tested in Casablanca wells to enhance oil production, such as acid tunneling in 2008, successfully implemented for the first time on an offshore well.
To avoid drilling into the oil-water contact and thus seeking to minimize water production, most of the Casablanca production wells have a short penetration into the reservoir with scarce cores and log coverage. As a result, the overall characteristics of the Casablanca reservoir are poorly known. Multiple reserves determinations and simulations by operators, partners and consultants have been carried out during the producing life of the field. Most of them failed in predicting field performance and led to underestimation of recoverable reserves. Therefore, the estimated ultimate recovery had to be based on individual well productivities and production decline curve data. A study sponsored by the European Union concluded in 2008 that Casablanca was an unsuitable candidate for future CO2 storage, mainly due to the lack of a reliable reservoir geological model, a mandatory requirement to secure underground storage approval.
‘We’ll Always Have Casablanca’
The estimate of recoverable oil evolved significantly over time, from 90 million barrels shortly after the discovery, well under the cumulative production to-date of 148 million barrels of oil, illustrating the very high complexity of the geology and becoming a good example of positive reserve “growth.”
After several changes in percentages and partnerships, the Casablanca field is currently operated by Repsol (68.67 percent) in a joint operating agreement with CNWL (14.47 percent), Petroleum Oil and Gas (9.46 percent) and Cepsa (7.40 percent). Today, after 40 years of first oil, the field is producing daily some 900 barrels of oil and 35,000 barrels of water from four active wells. The Casablanca platform also gathers, treats and exports the production coming from satellite fields such as Rodaballo, Boquerón, Chipirón, all named after local fish, and the tie-back of the deeper water completions of Montanazo and Lubina, both brought on stream in late 2012.
Since the mid-1990s, Casablanca’s production appears to be exhausted and close to an end, but the incorporation of new production, together with the recent facilities upgrades to meet the highest safety and environmental standards, including the development of the world’s first safety system for early leak detection in offshore operations, has steadily extended the Casablanca platform’s life since it was commissioned back in 1981. Additionally, the renewal of the exploitation license in December 2018 for 10 more years will make Casablanca reach a total of 50 years of production. Thus, it appears that we will still have Casablanca for a while!
Historical Highlights is an ongoing EXPLORER series that celebrates the “eureka” moments of petroleum geology, the rise of key concepts, the discoveries that made a difference, the perseverance and ingenuity of our colleagues – and/or their luck! – through stories that emphasize the anecdotes, the good yarns and the human interest side of our E&P profession. If you have such a story – and who doesn’t? – and you’d like to share it with your fellow AAPG Members, contact Matthew Silverman at firstname.lastname@example.org.