Twenty years ago the company performed its first hydraulic fracturing on the testing ground of Noyabrskneftegaz’s Karamovskoye field. Since then, the hydraulic fracturing technique has become ever more sophisticated: today it allows Gazprom Neft to not only stimulate production in Gazprom Neft fields, but also focuses on the development of hard to recover reserves.
The search for technologies for oil production stimulation started back in the late 19th century in the United States, as soon as the outstanding commercial perspectives of the oil industry became obvious. The low efficiency of the drilling equipment and production methods would then be compensated by exploding nitroglycerin inside wells. The idea was basic: the technique simply facilitated the destruction of the formation in the bottom-hole area and maximized fluid influx. However, this method turned out to be dangerous and rather crude.
The next step was bottom hole acidizing to dissolve lime stone cementing rocks in certain oil reservoirs. The first acidizing experiments were carried out in 1895. Only 30 years after the method was commercialized, the high efficiency of overpressure acid injection became evident. This initiated the development of fluid stream pressure based hard rock fracturing technique. The Americans are considered to be pioneers in hydraulic fracturing. The late 1940s are distinguished by the first successful hydraulic fracturing conducted by Halliburton and the first theoretical work by an American engineer Clark,* who described the technique and theorized the process in the well. Positive results achieved due to hydraulic fracturing quickly made the technique popular in the oilfields of the United States. Despite that the technique was not well studied and had imperfections, the total number of hydraulic fracturing jobs in the USA wells reached 100,000 by 1955.
In the Soviet Union, the first hydraulic fracturing jobs were performed in early 1950s. Notably, it is Soviet scientists who pioneered investigations enabling to model hydraulic fracturing process and predict results. The founder of the Moscow Institute of Physics and Technology, Academician Sergey Khristianovich, in cooperation with his colleagues, developed a theory of two-dimensional fracture creation and propagation in the formation. Their achievements are still used in predictive modelling. The peak of hydraulic fracturing in the USSR was in 1958-1962 when the number of fracturing jobs exceeded 15,000 a year. When large high-output fields were discovered in the Western Siberia, the hydraulic fracturing technique was no longer used as “easy” oil did not require additional simulation. Russia returned to the hydraulic fracturing technique only in late 1980’s when the oil & gas reserves structure had significantly changed.
In Search of Better Opportunities
For the decades the lack of demand has unfortunately resulted in Russian hydraulic fracturing equipment and experience being far behind the world level, and now hydraulic fracturing in Russian oil fields is utilized exclusively by foreign service companies.
Notwithstanding the changed market situation, all new technology trends are sourced from abroad. The main strategy is to reduce cost and increase efficiency of the technique, in addition to finding utilization methods in the most challenging conditions such as development of unconventional resources.
Hydraulic fracturing can be defined as a set of consecutive operations: identification of a fracture point to create fractures in the rocks of the oil formation, creation of conditions (perforations) to apply pressure to the formation in the chosen well sections, injection of the fracturing fluid under high pressure into the formation, injection of the propping agent (proppant) into the fracture, flushing of the well and its further operation. Since the first hydraulic fracturing took place, all the above-mentioned stages have undergone certain changes in order to tailor the technique to each oil field. Today, despite of extensive use, hydraulic fracturing is a case-specific technique ensuring optimal efficiency.
The first hydraulic fracturing jobs utilized water fracturing liquid, and river sand as a propping agent. Hydraulic fracturing was performed in wells to increase the production rate and no preliminary study of possible impact was conducted. Modern computer aids to process geologic data and model formation make it possible to select the most suitable area to initiate a fissure. Subsequent modelling that considers the formation rocks properties, allows calculation of required parameters of the fracturing fluid and selection of suitable proppant to achieve a fracture of optimal size and maximum conductivity.
“Gazprom Neft hydraulic fracturing development focused on finding the optimal composition of fracturing fluid and types of proppants” says Ildar Faizullin, Head of Hydraulic Fracturing Design Department of Gazprom Neft Research and Development Centre, “Gel injected into a well should be viscous enough to not be absorbed by the formation and carry the proppant to the fracture without losses avoiding its settling in the well. Yet the fluid should easily leak out from the fracture so not to affect its conductivity”. According to the expert, for this to occur, special substances, i.e. fluid viscosity reducing breakers, are added to the gel. Encapsulated breakers used today, rupture under pressure inside the fracture. The gel liquefaction starts only after the fracture is induced and stabilised. In addition to breakers, fracturing fluid may contain other special components to, among other things, reduce fluid friction on passing through a pipe. This allows reducing power expenditures. Selecting a proppant can also be tricky; the selection evolved from regular river sand into burnt clays and bauxites pellets. The aim is to achieve a propping agent price-strength-conductivity balance in view of the particular geological conditions.
Every year hydrocarbon reserves in easy formations are reduced and the latter are replaced with low-permeability formations featuring high heterogeneity, low poroperm properties and heavy compartmentalization. This adversely affects hydrocarbon production rate.
One of the most efficient methods to increase well productivity penetrating such formations is hydraulic fracturing, which may maximize oil recovery rate. After hydraulic fracturing, the well is better linked with natural fractures and high-permeability areas; formation area drained by the well increases.
The most widespread technique is multi-stage hydraulic fracturing in horizontal wells. It multiplies the production well flow rate. Today we are also developing unique technologies, one of which is multilateral wells where multi-stage hydraulic fracturing is conducted in each wellbore. The first in Russia bilateral well with multi-stage hydraulic fracturing is currently being drilled in Krayneye Field. Moreover, the technology of repeated multi-stage hydraulic fracturing is now under intense investigation and their usage will become relevant in several years.
Head of Geology and Development Department
Today it is strange to hear that formation hydraulic fracturing can be done to only pass a bottom-hole area damaged by remaining drilling mud and to link clean formation and a well. Twenty years it was typical when in high-permeability formations the drilling mud contaminated (colmataged) a rather large area around the well, preventing oil production. Nowadays there are almost no high-permeability formations left and the hydraulic fracturing purpose is to increase the oil recovery rates by covering a larger production zone, achieving a cost-effective production of tight reservoirs with low permeability and porosity.
New challenges also require a new approach to the technology implementation. During the first hydraulic fracturing jobs no more than 5 – 10 tons of the proppant were injected into a well, while today the amount is hundreds of tons. A large amount of the proppant is required when creating long fractures through a considerable part of the formation. To achieve such injection performance, powerful pumps, accurate calculation of fracture geometry and suitable fracturing fluid are required. For the chemists, the selection of an appropriate liquid is the main concern. It is no exaggeration to say that at least 60% of a successful hydraulic fracturing depends on the appropriate
The company conducted its first hydraulic fracturing in directional wells only. In the early 2000s it was decided to test efficiency of hydraulic fracturing in horizontal wells, however, the horizontal wells were drilled in rather dense formations and at high-permeability sections in conventional fields, where no significant complications were encountered. In the wells not initially designed for the technique, hydraulic fracturing purpose was to increase production that had been reduced due to natural productivity loss caused by colmatage of bottom-hole area by both particles of rock matrix and contaminants left from workovers. Moreover, unsuccessful hydraulic fracturing could make the situation worse, for example the fracture linking the formation and water-flooded sections. Despite all the above-mentioned, the first hydraulic fracturing trials in horizontal wells were quite successful allowing for more confidence when introducing multi-stage hydraulic fracturing in horizontal wells in low-permeability reservoirs.
Wide use of the multi-stage hydraulic fracturing technique started in the early 21st century in America when the first true success was achieved in the shale oil & gas fields. The multi-stage hydraulic fracturing triggered what has become know as “the shale revolution”. In Russia use of the technique started in the 2010s. Gazprom Neft selected Vyngapurovskiy field as a trial site, as it was a field where remaining reserves could not be recovered by conventional methods. The pilot project four-stage hydraulic fracturing was performed in the field in 2011.
Unlike the conventional hydraulic fracturing, the multi-stage technique utilizes special equipment which is lowered into the well during its completion. The equipment range is wide and should be selected taking into account the formation conditions as well as economic feasibility.
“In the beginning of multi-stage hydraulic fracturing in horizontal wells we used BHA with disposable sleeves and composite balls as isolators. (see chart)”, says Ildar Faizullin, “The borehole annulus was isolated by swellable packers, kind of plugs which swell when exposed to the oil. Packers sectioned the annulus where the fracturing fluid with the proppant could penetrate during staged hydraulic fracturing. Today we cement the borehole annulus. This is more complicated and expensive, but enhances reliability of hydraulic fracturing and control of the fracture initiation point”.
In 2014 the number of multi-stage hydraulic fracturing in horizontal wells of Gazprom Neft increased up to 168 jobs per year. As well as quantity, quality also improved. Today 10-stage hydraulic fracturing is a routine, and the record of 19-stage hydraulic fracturing had been reached in the Yuzhno-Priobskoye field of Gazpromneft-Khantos in the end of the year.
The latest technology development is BHA systems with reusable sleeves and packer as the isolator for fractured areas (see chart). The mechanically activated packer replaces conventional composite packer ball allowing a maximum number of fracturing stages limited only by well length and cost estimates. Equipment to open a sleeve with installed packer is lowered into the well on coiled tubing. Gazprom Neft used this hydraulic fracturing technique for the first time in the Priobskoye field and thus reached 15 fracturing stages with a prospect for further growth.
Believe it or not, we cannot say that developing hydraulic fracturing technique is becoming more complicated. Of course, particular stages require more sophisticated equipment, for example, fracture propagation modelling, secondary well testing to obtain more accurate information and analyse hydraulic fracturing, i.e. seismic survey, logging. On the other hand, heavy-duty pumps are designed for less compounded fracturing fluids and high pumping rate allows for low fluid viscosity, moreover, sometimes it is necessitated for successful hydraulic fracturing, for example multi-stage fracturing in low-permeability reservoirs such as in the Bazhenov formation.
Oil deposits in the Bazhenov formation are very promising nowadays for the Russian oil industry. Gazprom Neft endeavors to find an optimal development method for the challenging reserves. It is obvious that the principle technique here is multi-stage hydraulic fracturing with optimal parameters, which are yet to be found. The first multi-stage hydraulic fracturing in the Bazhenov formation showed low efficiency of conventional fluids and BHA systems. Only narrow-width fractures are possible to form in the hard rocks of the Bazhenov formation and fracturing gel of standard viscosity settled in such fractures forms polymeric film resistant to washing away. The solution is to use low-friction water or even slickwater as a fracturing fluid.
However this water is now no longer used in hydraulic fracturing. The reason is simple: due to low viscosity the water does not deliver the proppant to the fracture, proppant settled in the well does not contribute to the fracture formation but hinders the operation. Today heavy-duty pumps are used to avoid this: proppant has no time to settle inside the well due to super-high injection rate. This concept was the one selected for the Bazhenov formation. The higher is fluid flow rate, the higher the pipe wall pressure is. Large bore pipes should be used to maintain the allowable pressure. Therefore, hydraulic fracturing BHA systems with sleeves and production tubing could not be used in the Bazhenov formation.
“The new-design ten-stage hydraulic fracturing was first conducted in Palyanovskoye Field of Bazhenov formation in December, 2015.” says Mikhail Cherevko, Chief Geologist of Gazprom Neft Khantos,“We used the plug and perf method of hydraulic fracturing when hydraulic fracturing stages are separated with special plugs lowered into the well on coiled tubing, and proppant is injected at each hydraulic fracturing through the perforated channels. By this technique it was possible to form branched conjugated fractures and define and monitor the direction thereof”. In the West this technique has been successfully used for about ten years already and it is called “plug and perf method”. Jet perforation with no sleeves required and several holes made per each fracturing stage make it possible to form a fracture network vs. a single main fracture formed by the conventional hydraulic fracturing technique. The tubing is not lowered into the well but the fracturing fluid is injected directly through production string, while fracturing stages are separated by special composite plugs.
Time will tell how efficient such multi-stage hydraulic fracturing technology will be. “By now two wells in Russia have been drilled by the plug and perf method, and both were successful”, Aleksandr Milkov, Head of Bazhenov Project Well Completion Division of Gazprom Neft, shares with us. “We hope that the result will be also successful”.
However, the search for new solutions continues, and there is still definitely a room for improvement. According to Aleksandr Milkov, the future is in mobile equipment, injection speed increasing and simplification of hydraulic fracturing gels chemical composition; and, in general, in low-cost and efficient solutions.
Sofia Zorina, Gazprom Neft