Acid-tunneling is an acid jetting method for stimulating carbonate reservoirs. Several case histories from around the world were presented in the past showing optimistic post-stimulation production increases in open-hole wells, comparing to conventional coiled tubing (CT) acid jetting, matrix acidizing, and acid fracturing. However, many questions about the actual tunnel creation and tunneling efficiency are still not answered. In this paper, the results of an innovative full-scale research program involving water and acid jetting are reported for the first time.
The tunnels are constructed through chemical reaction and mechanical erosion by pumping hydrochloric (HCl) acid through conventional CT and a bottom-hole assembly (BHA) with jetting nozzles and two pressure-activated bending joints that control the tunnel initiation directions. If the jetting speed is too high and the acid is not consumed in front of the BHA during the main tunneling process, then unspent acid flows toward the back of the BHA and creates main wellbore and tunnel enlargement with potential wormholes as fluid leaks off, lowering the tunneling length efficiency.
Full-scale water and acid jetting tests were performed on Indiana limestone cores with 2-4 mD permeability and 12-14% porosity. Many field-realistic combinations of nozzle sizes, jetting speeds, and back pressures were included in the testing program. The cores were 3.75-in. in diameter by 6-in. in length for the water tests, and 12-in. in diameter by 18-in. in length for the tests with 15-wt% HCl acid. The jetting BHA was moved as the tunnels were constructed, at constant force on the nozzle mole, to minimize the nozzle stand-off distance. Six acid tests were performed at the ambient temperature of 46F and two at 97F. The results from the acid tests show that the acid tunneling efficiency can be optimized by reducing the nozzle size and pump rate. The results from the water and acid tests with exactly the same parameters to match the actual CT operations in the field show that the tunnels are constructed mostly by chemical reaction and not by mechanical erosion. The acid tunneling efficiencies obtained from the full-scale acid tests are superior to the average tunneling efficiency of more than 500 actual tunnels constructed during more than 100 acid tunneling operations performed to date worldwide.
The paper describes the full-scale water and acid jetting tests on Indiana limestone cores. The major novelty of this test program consists of performing all measurements with back pressure, unlike all previous water and acid jetting studies reported in literature, to more accurately mimic the downhole well conditions. The novel understanding of the combined effect of the nozzle size, pump rate, and back pressure significantly improves the actual acid-tunneling efficiency.
The objective of this work is to present the utilization of high-power laser technology in different downhole flow enhancement applications. High power laser technology provides innovative non-damaging technology and is an alternative to several current downhole conventional technologies. Wide ranges of high power laser applications have been identified, evaluated and successfully tested in the laboratory, these applications are related to improve production. The precision of controlling the laser parameters enables the properties flow improvement in all rock types.
Laser energy generated and transmitted from the laser source to the target via fiber optics cables. The laser beam is controlled by downhole tool and optical bottom hole assembly. Based on the identified applications, the laser tools are designed and built combining mechanical and optical components that are aligned and assembled accordingly, the tools released the final shaped beam to the target. The controlled beam generates thermal energy, this energy can melt, vaporize or spall the formation, depending on the application needed.
High power laser technology has the potential to change the industry in several downhole applications including perforations, heating, fractures initiation, open hole notching, deep perforation, and drilling. The motivation for searching for alternative technologies is the cost effective and advancement of the laser technology and the need for none damaging environmentally friendly alternative technology. Laser provides unique advantages for downhole applications, such as accuracy, precision, and power, these parameters have been successfully tested in the lab and the optimized setting are configured in the tool. Several iterations of the tools have been done to optimize and finalize the successful design. The tools are designed to fit in slim holes as small as four inches. In addition, the tools are designed to operate in a fluid environment, the tools are equipped with purging capabilities to circulate gas or fluid, the functions of the purging are to clean the hole from the debris and carry the cuttings.
The technology provides very small footprint and is environmentally friendly, it is a waterless technology when it is used for fracturing, and a non-explosive perforation base technology when it is used as a perforation gun. The unique futures of the technology are the precision is controlling and orienting the energy in any direction regardless of the reservoir stress orientation and magnitude. The precision in orienting the energy enables to focus the energy on the pay zones directly maximizing reservoir contact.
Establishing communication between the wellbore and hydrocarbon-bearing formations is critical to ensure optimal production. Laser is a new technology that utilizes the power of light to perforate rocks. The technology is non-damaging, safe (non-explosive), and affords precise control over the perforation's geometry (size and shape). The process creates an enhanced tunnel that improves the flow and increases production. The technology has been successfully demonstrated in the lab environment. The results are used to develop a field deployment strategy. In the field, the laser source will be mounted on a coiled tubing unit on the surface and transmitted downhole via optical fibers. Downhole, the beam is out-coupled and directed to the target using an optical bottom hole assembly (oBHA). This tool combines optical and mechanical components to control the beam and produce multipole shots per foot as needed to create the desired perforation network. High-power laser perforation is the next new intelligent perforation generation that will change current well perforation.
Laser-rock interaction drives in the transformation of electromagnetic energy into thermal energy. This results in a highly localized and controllable temperature surge that can melt or vaporize the rocks. These properties make the technology a unique alternative to current perforation techniques based on shaped charge guns. The thermal process induced by the laser enhances the flow properties of the rock, especially in tight formations. Laser perforation has been tested on all types for rocks including unconventional tight sands. This has been proven through extensive pre- and post-perforation characterization over the last two decades.
This work presents the development and evolution of the high-power laser tools for subsurface applications. These tools provide innovative and non-damaging alternatives to current downhole technologies. In the lab, the laser technology has been proven to improve the flow properties; thus, it can improve communication between the wellbore and formation. To achieve this efficiently in the field, it is necessary to develop different tool designs and configurations, manufacture prototypes, conduct extensive tests, and optimize each part before upscale for field operations.
The laser source is mounted in a coil tubing rig at the surface; the coil contains the optical fiber cable used to convey the energy to the downhole tool. The tool combines mechanical and optical components to transform, control, and direct the laser beam. The design and configuration of each tool assembly varies depending on the targeted application. For example, the perforation tool converts and splits the beam into several horizontal beams; whereas the drilling tool emits a straight beam with controlled size for deeper penetration. They also incorporate purging capabilities to circulate fluids to clean the hole from the debris and carry the cuttings. The entire assembly must be made to fit in slim holes as small as four inches. And finally, ruggedized to operate in a complex environment with high pressure and temperature.
The technology improves reach and provides versatility in a compact and environmentally friendly manner. For example, it is a waterless technology when it is used for fracturing, and a non-explosive based perforation when it is used to perforate. The unique features of the technology enable a precise, controlled, and oriented delivery of energy in any direction, regardless of the reservoir stress orientation and magnitude. Thus, it enhances reach to produce from pay zones that are bypassed by current conventional technologies and practice. The motivations to search alternative technologies are the advancement of technologies, including high power lasers, and the need to enhance several applications in deeper wells in an environmentally friendly manner.
Considering the important role that perforation laboratory testing can play in establishing field completion strategies, and thus ultimately well performance, efforts are currently underway to further strengthen the link between laboratory results and field well performance predictions. Some of these efforts focus on integrating advanced diagnostic and computational tools (namely computed tomography (CT), and pore-scale flow simulation) into the perforation testing workflow. This integration enables local variations in permeability and porosity to be identified and quantified, thus improving the interpretation of perforation laboratory results, and ultimately the translation of these results to the downhole environment.
CT techniques have been used for core analysis, characterization, and flow visualization since the early 1980s. By the early 1990s, these techniques were being applied to the investigation of laboratory-perforated cores to enhance the interpretation of tests conducted following API RP19B Section 2 or 4. This application has increased dramatically since 2012, following the installation of a CT scanning system on-site at a perforating laboratory facility. As a result, this non-destructive technique has become a preferred method to routinely characterize perforation tunnels and the surrounding rock, as well as to enable the repeated inspection of a perforated core at multiple steps throughout a test sequence designed to mimic field operations scenarios. Coinciding with this development has been the advancement and application of micro-CT technology to better understand pore-scale phenomena, both near and away from the perforation.
This paper introduces an integrated test program currently underway and summarizes key results from two experiments in which stressed rock targets were perforated under significantly different conditions. The first experiment involved perforating a moderate strength sandstone core under conditions that retained substantially all perforation damage, thus preserving the "crushed zone". Micro-CT analysis of different locations within the crushed zone region revealed significant compaction, with porosity reductions ranging from 10 to 50% below that of the native rock. Permeability at one of these selected locations was determined and found to be reduced by approximately 35% below the native rock value. The second experiment involved perforating a very high-strength sandstone core under conditions intended to produce full cleanup. CT and micro-CT analysis revealed fine fractures near the tunnel tip and confirmed the near-complete removal of the perforation damage, with only a very thin (less than 1 mm) compacted zone remaining at the tunnel wall. Although this region is interpreted to have very low permeability (as indicated by the near-zero connected porosity detectable at the resolution investigated), a fracture network combined with the shell’s minimal thickness suggests that this would provide a minimal impediment to inflow.
Ongoing work aims to expand these findings and capabilities. A main effort going forward centers on simulating core-scale perforation inflow, incorporating the localized rock property variations determined as described in this paper. Additional property variations away from the perforation (for example, natural heterogeneity and/or anisotropy that often exist in reservoir wellcore samples) will also be taken into account. Such localized variations, both near and away from the perforation, are generally not taken into account in typical Section 4 test programs. Consequently, this ongoing effort will ultimately strengthen the relevance of Section 4 results to the downhole environment.
In recent years, numerical tools increasingly have been used in conjunction with experiments to provide better insight into the flow characteristics of perforated cores and perforated well-scale formations. Several numerical studies on perforation fluid flow have been conducted for core scale. However, comprehensive details relating to the modeling of perforation-zone damage and thickness, flow directionality, debris mechanisms, and implications for cleanup have not been studied in detail. In addition, most computational fluid dynamics (CFD) studies have used traditional Navier-Stokes-based solvers. In this study, the authors have used a commercially available flow-simulation software based on the lattice Boltzmann method (LBM) to calculate the complex flow and cleanup mechanisms around the perforation tunnel.
A coiled-tubing (CT)-acid-tunneling-stimulation technique has been successfully applied in the preceding 15 years on limestone and dolomite reservoirs around the world (the Middle East, southeast Asia, North America, South America, and Europe). Several case histories were presented in the past showing that this technique might bring significant benefits over other carbonate-stimulation methods in openhole wells. In this paper, the parameters affecting the predicted and achieved tunnel lengths are discussed for the first time. The acid-tunneling technique consists of pumping hydrochloric acid (HCl) through conventional CT and a bottomhole assembly (BHA) with jetting nozzles to create (without drilling) stable drainage holes (tunnels) into the reservoir pay zone. The BHA also includes a special kickoff tool, with two pressure-activated bending joints, that controls the tunnel-creation direction. The acid that is not consumed during the main tunneling process leaks into the reservoir rock, creating wormholes that improve the connectivity between the reservoir and the wellbore and positively influence well production. This acid-tunneling technique can potentially create numerous tunnels with different depths. The optimization of the tunnel-creation-depth selection is made by production-software simulation using such critical information as the well parameters (trajectory and size), available logs (image, resistivity, caliper, drilling), and past reservoir information.
The results from many field case histories involving the CT acid-tunneling technique from around the world were presented previously. However, many questions remain unanswered regarding the actual downhole tunnel-initiation/creation process. In this study, a detailed discussion of acid-tunneling modeling is included to answer some of those questions. The parameters affecting the predicted tunnel lengths and the parameters that could be monitored or adjusted to create the tunnels smoothly are discussed. This paper describes the CT acid-tunneling technology and discusses some of the most important questions regarding downhole CT acid-tunneling creation. The acid-tunneling-technique performance and benefits confirmed during field operations are presented.
Wehunt, C. Dean (Chevron North America Exploration and Production Company) | Lattimer, Stefan K. K. (Chevron Europe, Eurasia, and Middle East Exploration and Production Company) | McDuff, Darren R. (Chevron Energy Technology Company)
In this paper, we provide an update on recent advances for and summarize global experiences with dendritic-acidizing (DA) methods, or acid tunneling. We include both coiled-tubing (CT) deployed methods and non-CT methods, and discuss process limitations, candidate-selection criteria, job-design factors, operational learnings, risks, and surveillance requirements and opportunities. A comprehensive review of published information is provided for three different tunneling methods along with relevant information for several other tunneling methods. This literature information is supplemented by depth, temperature, and pressure records for the three processes, which are discussed in detail. Execution factors such as logistics required, length of time required, and volumes of acid and other fluids used are also compared for the three methods.
Previous papers have focused on only one of the methods, whereas we will discuss acid-job optimization, process risks, and surveillance requirements for multiple acid-tunneling methods in substantially greater depth than have past authors. The three methods detailed in this paper are all viable but may have different niches. Differences in the job counts for the different methods are easily explained by differences in process vintages, execution speeds, and depth limitations. Previous optimization efforts were focused on tunnel creation but not acid-job effectiveness in terms of the wormholes generated adjacent to the tunnels; however, some progress is now being made in that regard. There are differences in the processes regarding pushing or pulling the jetting nozzles into the tunnels, and differences in resulting tunnel trajectories. Prejob caliper data are more critical for one of the processes than for the others, and there are significant differences in ability to measure or control tunnel direction. The tunneling tools have different sizes, but when toolsize alternatives are available, the larger tool sizes offer no clear advantages to the operator. Useful risk-mitigation measures are also discussed, and a comprehensive bibliography is included to facilitate further examination of the technology alternatives by other petroleum-industry professionals.
Nunez, Alvaro Javier (Petroleum Development Sultanate of Oman) | Molenaar, Mathieu (Petroleum Development Sultanate of Oman) | Al-Salmi, Masoud (Petroleum Development Sultanate of Oman) | Al-Farei, Ibrahim (Petroleum Development Sultanate of Oman) | Gheilani, Hamdan (Petroleum Development Sultanate of Oman) | Sayapov, Ernest (Petroleum Development Sultanate of Oman) | Al-Shanfari, Abdul Aziz (Petroleum Development Sultanate of Oman)
Deep exploration and gas wells with very tight formations to be hydraulically fractured in the Sultanate of Oman, have been preferably perforated with Abrasive Jet perforating systems in order to reduce the perforating skin and to be able to break down the formation compared to the regular deep penetration shaped charges which historically have been unsuccessful used. The complexity of the Abrasive Jet perforating operations requires Coil Tubing, Sand Management System (SMS), specialized perforator BHA, correlating devices, pumping and mixing equipment and a substantial quantity of fluids and sand per interval.
Perforating guns with reactive liner technology, conveyed with electric line, have been considered for these types of wells. This technology delivers enhanced perforations facilitating fracturing operations by reducing the break down pressures. Initially well data from three exploration wells "A", "B" and "C" were extensively evaluated with very high chances to succeed. The job execution was planned parallel to the Abrasive Jet perforation in case the formation would have not broken down after the gun perforations. In well "A" two stages were executed successfully using the reactive liner shape charge technology while in the well "B" the first stage was neither broke down after reactive liner charges nor after Abrasive Jet perforations; the second stage in well "B" was successfully executed after the reactive shape charge gun perforations. This initial success of these two exploration wells was replicated in another gas field also with tight formations which historically has been perforated with Abrasive Jetting near well "C".
As a result of this, the efficiency, the perforating accuracy and the cost savings are substantial improved; the HSE exposure is much less due to the fact that less resources are needed compared to the requirements for Abrasive Jet. This paper will go thru the design, planning, execution, results and the big step change in the fracturing operations in these types of wells in which Abrasive Jetting is not always the best preferred option.
With an increasing focus on identifying cost-effective solutions to well design with a minimal impact on productivity, this paper will focus on an alternative to cesium (Cs) formate as the perforation fluid in the high-pressure/high-temperature (HP/HT) Gudrun Field operated by Equinor. Cs formate has been used with success for drilling and perforating many HP/HT wells. However, because of the significant cost of this fluid coupled with low oil prices, Equinor wanted to perform testing to assess the performance of an alternative oil-based mud (OBM) as a perforation fluid. In this paper we describe the extensive qualification testing that we have conducted, which includes coreflooding using representative plugs from Gudrun Field under downhole temperature and pressure conditions. In addition, eight API RP19B (2014) Section IV perforation tests have been conducted to compare the performance of the Cs formate with the OBM. These tests were undertaken using gas- and oil-saturated cores to reflect different production scenarios. The main aspects of the perforation operation that were reflected in the test design were as follows:
On the basis of the results of the coreflooding combined with the API RP19B (2014) Section IV testing, the OBM was selected as the perforating fluid for use on Gudrun Field. The perceived benefits of using the OBM were as follows:
Perforation modeling is described, and a comparison is made between this and the API RP19B (2014) Section IV tests. Finally, the well-startup experiences and the production data are presented, demonstrating the effectiveness of the OBM as a perforation fluid.
At present, along with conventional energy sources continually consumed, renewable energy sources are increasingly favored, especially the clean and inexhaustible geothermal resources have been universally valued both at home and abroad. In particular, the Enhanced Geothermal Systems (EGS), which is mainly aimed to exploit the thermal energy of Hot Dry Rock (HDR) at depths of 3 to 10 kilometers underground, has been full of interest to many countries. However, so far there hasn't been an EGS being successfully put into commercial operation because of its shortcomings such as small scale, low efficiency, etc. In this article, in response to the bottleneck of the study on the development of traditional EGS based on drilling technology (EGS-D), a conceptual model of EGS based upon excavation technology (EGS-E) is innovatively proposed and its main components of underground structure are described in this paper. As for ‘High ground stress, High ground temperature and High osmotic pressure’ initial conditions with regards to deep rock mass, the excavation experience, which is worth being learnt from extensive review of previous study as well as practical experience such as the successful excavation of ultra-deep mines in the gold field of South Africa, is summed up. The underground spatial structure that may be reasonable to the so-called EGS-E is being tried establishing. It is expected to provide with a basis for our subsequent numerical modeling.
Currently, seeking and developing clean new energy is the basic energy exploitation strategy, and the clean and inexhaustible geothermal resources have been universally valued both at home and abroad. Geothermal energy is the heat energy mainly generated by the transmutation of radioactive elements in rocks, which is 2.0934×1018 kJ annually. And the geothermal energy stored at depths of less than 10 kilometers underground was estimated to be 170 million times the amount of heat released from all the coals stored in the earth by Pollack and Chapman in 1977 (Wang Ruifeng, 2002). It can be seen that the reserves of geothermal energy are very considerable.
In spite of its advantages of stability, continuity and high utilization coefficient, the scale of the geothermal energy with temperature less than 150 °C at depths of less than 3 kilometers underground is usually too small to maintain the demand for long-term stable electricity production which is mainly hydrothermal and only accounts for 10% of all the geothermal energy stored in the earth (Guo Jian et al., 2014). Therefore, the enhanced geothermal system (EGS) which aims at exploiting the geothermal energy from hot dry rock (HDR) at depths of 3 to 10 kilometers has gradually attracted people's attention.