Cement is a key element for successful drilling and completing of a well. From oil and gas wells to geothermal applications, cement is a major material ensuring zonal isolation. With an increase in global energy needs and an expected uptick in drilling and plugging and abandonment activities, evaluating and understanding cement properties is crucial, since these properties are used in various engineering designs and calculations. The objective of this paper is to present how Nuclear Magnetic Resonance (NMR) can be used to understand the cement hydration process and the development of key properties such as strength and porosity. NMR applications for cement include determination of porosity, water interactions, identification of hydration stages and C-S-H gel development with curing time. Since water is present in all cement slurries, NMR can potentially help to understand microstructural changes in cement during curing. Data from more than 600 cement specimens cured for more than a year are compiled. Standard cement properties such as UCS (unconfined compressive strength) are compared with NMR responses. In this paper, we document cement hydration and porosity changes through NMR measurements in samples with five different recipes. Our study also confirms a strong correlation between NMR response and cement strength.
The goal was to search for a replacement of CaCl2 which presents the most widely used accelerator for oil well cement used in cold and arctic environments and sometimes in deepwater drilling. For this purpose, novel calcium silicate hydrate (C-S-H) nanoparticles were synthesized and tested. The C-S-H was synthesized by the precipitation method in an aqueous solution of polycarboxylate (PCE) comb polymer which is widely used as concrete superplasticizer. The resulting C-S-H-PCE suspension was tested in the UCA instrument as seeding material to initiate the crystallization of cement and thus accelerate cement hydration as well as shorten the thickening time at low temperature. It was found that in PCE solution, C-S-H precipitates first as nano-sized droplets (Ø ~20 - 50 nm) exhibiting a PCE shell. Following a rare, non-classical nucleation mechanism, the globules convert slowly to nanofoils (HR TEM images: l ~ 50 nm, d ~ 5 nm) which present excellent seeding materials for the formation of C-S-H from the silicate phases C3S/C2S present in cement. Thickening time tests performed at + 4 °C in an atmospheric consistometer revealed stronger acceleration than from CaCl2 while very low slurry viscosity was maintained, as was evidenced from rheological measurements. Accelerated strength development was checked on UCA cured at + 4 °C and under pressure, especially the wait on cement time was significantly reduced. Furthermore, combinations of C-S-H-PCE and HEC as well as an ATBS-based sulfonated fluid loss polymer were tested. It was found that this C-S-H- based nanocomposite is fully compatible with these additives. The novel accelerator based on a C-S-H-PCE nanocomposite solves the problems generally associated with CaCl2, namely undesired viscosity increase, poor compatibility with other additives and corrosiveness against steel pipes and casing.
With the current applications of CO2 in oil wells for enhanced oil recovery (EOR) and sequestration purposes, the dissolution of CO2 in the formation brine and the formation of carbonic acid is a major cause of cement damage. This degradation can lead to non-compliance with the functions of the cement as it changes compressive and shear bond strengths and porosity and permeability of cement. It becomes imperative to understand the degradation mechanism of cement and methods to reduce the damage such as the addition of special additives to improve the resistance of cement against acid attack. Hence, the primary objective of this study is to investigate the effects of hydroxyapatite on cement degradation.
To investigate the impacts of hydroxyapatite additive on oil well cement performance, two Class H cement slurry formulations (baseline/HS and hydroxyapatite containing cement/HHO) were compared after exposure to acidic environments. To evaluate the performance of the formulations, samples were prepared and aged in high-pressure high-temperature (HPHT) autoclave containing 2% brine saturated with mixed gas containing methane and carbon dioxide. Tests were performed at different temperatures (38 to 221°C), pressures (21 to 63 MPa) and CO2 concentrations (10 to 100%). After aging for 14 days at constant pressure and temperature, the samples were recovered and their bond and compressive strength, porosity and permeability were measured and compared with those of unaged samples.
The results demonstrated that adding hydroxyapatite limits carbonation. Baseline samples that do not contain hydroxyapatite carbonated and consequently their compressive strength, porosity, permeability, and shear bond strength significantly changed after aging while hydroxyapatite-containing samples displayed a limited change in their properties. However, hydroxyapatite-containing samples exhibit high permeability due to the formation of microcracks after exposure to carbonic acid at high temperature (221°C). The formation of microcracks could be attributed to thermal retrogression or other phenomena that cause the expansion of the cement.
This article sheds light on the application of hydroxyapatite as a cement additive to improve the carbonic acid resistance of oil well cement. It presents hydroxyapatite containing cement formulation that has acceptable slurry properties for field applications and better carbonic acid resistance compared to conventional cement.
Prabhakar, Abhinav (National University of Singapore) | Lee, Namkon (Korea Institute of Civil Engineering and Building Technology) | Ong, Khim Chye Gary (National University of Singapore) | Zhang, Minhong (National University of Singapore) | Moon, Juhyuk (Seoul National University) | Cheng, Arthur (National University of Singapore) | Kong, Kian Hau (National University of Singapore)
This study aims to design and evaluate a well cement slurry as an alternative to the standard API ‘G’ slurry for utilization in plugging and abandonment (P&A) of oil and gas wells. OWC slurries are formulated with Portland API ‘G’ cement as the base material, along with calcium sulfoaluminate (CSA) cement, gypsum and chemical additives. The slurries are experimentally tested using API standard procedures to determine gel transition time and right-angle-set (RAS) tendency at a temperature of 50 °C and pressures up to 34.5 MPa (5000 psi). The hydration characteristics of CSA cement can be utilized to control the gel development behaviour of a well cement slurry in order to minimize fluid or gas migration. Formation of ettringite greatly influences early age gelation. The potential to enhance gel strength development of an OWC slurry with CSA cement is presented whereby the gel transition time decreases with higher dosages of CSA cement. Thickening time studies to investigate the RAS tendency of the designed cement slurries are presented.
Ahdaya, Mohamed Saad (Missouri University of Science and Technology) | Imqam, Abdulmohsin (Missouri University of Science and Technology) | Jani, Priyesh (Missouri University of Science and Technology) | Fakher, Sherif (Missouri University of Science and Technology) | ElGawady, Mohamed (Missouri University of Science and Technology)
One of the most important steps in drilling and operation completion is oil well cementing to provide wellbore integrity. Cementing is usually performed in the oil industry using conventional Portland cement. Even though Portland cement has been used for many years, it has several drawbacks, including operational failures and severe environmental impacts. Fly ash based geopolymer cement has been recently investigated as a low-cost, environmentally friendly alternative to Portland cement. This research develops a novel formulation of Class C fly ash based geopolymer and investigates its applicability as an alternative to Portland cement in hydrocarbon well cementing. Twenty-four variations of fly ash Class C based geopolymers were prepared, and by comparing several of their properties using API standard tests, the optimum geopolymer formulation was determined. The ratios of alkaline activator to fly ash that were used are 0.2, 0.4, and 0.8, along with different ratios of sodium silicate to sodium hydroxide, including 0.25, 0.5, 1, and 2. Multiple sodium hydroxide concentrations were used, including 5, 10, and 15 molarity. The selection of the optimum formulation was based on five different tests, including rheology, density, compressive strength, fluid loss test, and stability tests (sedimentation test and free fluid test). Then, a comparison between the optimum mix design and Portland cement was conducted using the same tests. Based on our results, increasing sodium hydroxide concentration resulted in an increase in compressive strength and showed a slight decrease in the plastic viscosity. However, increasing in the alkaline activator to fly ash ratios increased plastic viscosity, and thus the pumpability of the slurry was reduced. Increasing the sodium silicate to sodium hydroxide ratio significantly decreased the fluid loss. The optimum design of geopolymer, which had lower fluid loss, 93 ml after 30 minutes, sufficient compressive strength, 1195 psi, and an acceptable density, 14.7 lb/gal, and viscosity, 50 cp, was selected. Compressive strength of the optimum design showed better results than neat Portland cement. Unlike neat Portland cement, which needs fluid loss additives, the new formulation of geopolymer investigated in this study showed fluid losses lower than 100 ml after 30 min when tested using a low-pressure, low-temperature filtrate loss tester. The higher mechanical strength of geopolymer using fly ash Class C compared to Portland cement is very promising for achieving long-term wellbore integrity goals and meeting regulatory criteria for zonal isolation. The rheological behavior, compressive strength, and fluid loss tests results indicate that fly ash Class C based geopolymer has the potential to be an environmentally friendly alternative to Portland cement when cementing oil wells.
Lu, Haichuan (CNPC Offshore Engineering Company Limited) | Zheng, Huikai (CNPC Offshore Engineering Company Limited) | Li, Zongyao (CNPC Offshore Engineering Company Limited) | Feng, Wangsheng (CNPC Offshore Engineering Company Limited) | Tang, Shaobing (CNPC Offshore Engineering Company Limited) | Zou, Jianlong (CNPC Offshore Engineering Company Limited, Key Laboratory of Drilling Engineering of CNPC) | Li, Lirong (CNPC Offshore Engineering Company Limited, Key Laboratory of Drilling Engineering of CNPC) | Tan, Wenli (CNPC Offshore Engineering Company Limited, Key Laboratory of Drilling Engineering of CNPC)
Lost circulation and fluid channeling in cementing process are common technical problems in domestic and overseas oil-gas field, and making huge damage to oil-gas field development. The cement sheath is easy to be broken because it has only little deformation under force which will affect the cement long-term packing property that could ensure the oil and gas production go smoothly. Aiming at the above problems, to conquer the shortage of traditional materials, a new multi-functional material is developed by combining nanotechnology and polymer synthesis technology according to microstructure design principle. And then an intelligent thixotropic cement slurry system is built. The novel intelligent thixotropic cement slurry is able to rapidly respond to the external stimulus of shear force due to the dynamic net structure between the nanometer material and the polymer. It is thin under shear and become thick rapidly when it is still and become thin again after shear. The process is reversible. The new system which conquers the problems existing in the conventional thixotropic cement slurry could not only make full use of its functions but also reduce the construction risk. The experiments show that the cement slurry has strong thixotropy unaffected by temperature and short transition time and can prevent lost circulation and fluid channeling in cementing process. And the cement stone with the multi-functional material has well tenacity and excellent ability of resisting damage which could keep long-term packing property. Besides, the thixotropic mechanism of nanometer material and its function on mechanical property improvement are studied in the point of microstructure, which will lay a certain foundation for the application of nanotechnology in well cementing. The intelligent thixotropic cement slurry have been used successfully in many oil fields, which prove its function on solving the problems of fluid channeling and lost circulation.
Li, Bodong (Drilling Technology Division, EXPEC Advanced Research Center) | Zhan, Guodong David (Drilling Technology Division, EXPEC Advanced Research Center) | Suo, Zhongwei (SINOPEC Research Institute of Petroleum Engineering) | Sun, Mingguang (SINOPEC Research Institute of Petroleum Engineering)
This paper introduces the hydro-efflux hammer - a rotary percussion drilling tool that is developed to improve ROPs in hard and abrasive formations. Hydro-efflux hammer is a hydro-mechanical tool which utilizes drilling fluid to power its continuous percussion motion. The tool's rock-breaking capability is achieved by the high unit load and unique breaking methods. The periodic impact generated by the tool excites strong pulsating stress waves, which strengthens the concentrated stress in internal rocks, and speeds up the rock breaking process. For the design of the tool, improved system integrity is achieve by implementing optimized actuating mechanism, sealing components, and anticorrosion materials. The optimization also involves fine tuning the tool including its percussion stroke and impact energy based on the formation characteristics. Recent field test results in challenging formations are presented and analyzed. In this work, based on the testing result, a number of approaches to extend the life time of the tool for higher performances are also proposed.
Al-Mohailan, Mohannad (Kuwait Oil Company) | Nellayappan, Karthikeyan (Kuwait Oil Company) | Patil, Dipak (Kuwait Oil Company) | Al-Qadhi, Fahad (Kuwait Oil Company) | Sounderajjan, Mahesh (Kuwait Oil Company) | Kunchur, Basavaraj (Napesco Cementing) | Hussain, Khaddar (Napesco Cementing)
In deep wells in Kuwait, completions are fairly standardized which after flow testing are handed over to assets. In course of time, well interventions were done due to one or more of the following reasons - water shut off, addition of zone, plugged perforations, tubing check or production logging. In course of these Rigless interventions over a period of two decades wells have accumulated with fish primarily coiled tubing or wire line fish with some wells being partially plugged. In view of this, a snubbing unit with targeted capabilities to fish inside tubing was deployed.
Earlier, the well could not be killed either by bull heading or with heavy mud circulated by coiled tubing. Thus, the well was identified as a candidate to mobilize a snubbing unit to clean out and place a cement plug. The Snubbing operations itself was being performed for the first time under a regular contract.
The operations include a complete cleaning of all the bridges suspected from about 4200 ft to the deepest depth possible as per the availability of the work string tubulars. It was identified from previous records, this is likely to be an off bottom kill with mud and may not effectively prevent the gas seepage. Thus a detailed planning for a critical cement plug to be placed inside the liner to isolate the open hole from below was made.
Extensive laboratory testing and job modeling was conducted to ensure proper placement of the cement slurry in a challenging HP/HT environment. A 16.5 ppg Gas Block Slurry with low Fluid Loss and favorable rheological propertieswas utilized. Additionally, an alternative, customized engineered designed spacer was used to prevent the formation of a filter cake.
The cement plug was placed with only 1.5″ ID workstring in a 5″ liner while taking due care to prevent cement back into the 31/2″ completion tubing above. Besides, care had to be taken to factor in a sufficiently long thickening time to enable pull out work string safely. The well was successfully secured and isolated and will now allow the utilization of a work over rig to recomplete the well.
The use of a Snubbing Unit has been proven effective to isolate the gas zone and ensure zero pressure on surface to enable mobilization of Workover rig. The paper discusses the challenges in design, planning and operations of placing a 8 bbl plug to stem the gas which made the well unapproachable for the last 3 years.
In 2010, a deep well in North Kuwait was facing continuous complications with gas cuts during completion operations. In order to secure the objectives of the well the open hole section was cemented and was stimulated in LM zone with no success in flow at surface. Additionally, heavy mud was pumped to control the gas influx, resulting in barite settlement but the well reflected pressure of 4000 psi. In order to subdue, attempts have been made to kill the well with CT and heavy mud which failed as CT could not be run to the deepest depth due to high circulating pressures.
Rock strength is an important property to measure for determining its effect on drilling, wellbore stability, and potential well completions associated with hydraulic fracturing of unconventional reservoirs. The industry traditionally relies on elastic moduli measured from core plugs to determine the stress anisotropy to predict the extent of hydraulic fractures. This provides some estimate of the expected stimulated rock volume in unconventional reservoirs. Rock strength however based on the finding of this study could also be a factor that needs to be considered for designing hydraulic fracturing plans to stimulate production from the rock volume. However, rock strength is difficult to measure in highly laminated source rocks comprising unconventional reservoirs. The existence of weak, horizontal bedding planes within the laminated rock fabric creates anisotropy that influences the rock strength values obtained. Moreover, drilling and extracting intact horizontal, vertical, and diagonal core plugs to test the effects of anisotropy on the rock strength is difficult to achieve. Often, the plugs fracture during extraction due to the laminated fabric. To compensate for the challenge of extracting intact core plugs from these lithofacies, this study proposes that rock strength can be estimated without the need of extracting core plugs. Instead, a new method is demonstrated where non-destructive rebound hardness measurements are collected across a specifically gridded, slabbed rock surface to provide an estimate of the rock strength. The collected rebound hardness values are converted into unconfined compressive strength values using an empirical algorithm. The empirical algorithm was developed using unconfined compressive strength values measured from core plugs correlated to rebound hardness numbers measured from the face of those same core plugs. The derived unconfined compressive strength values are then used to represent the source rock's mechanical characteristics which can be presented as a contour map across the surface. These results have been correlated to the mineralogy of the rock surface, quantified and mapped using micro-X-ray Fluorescence elemental maps. Differences in unconfined compressive rock strength can then be correlated to the changing mineral content of the rock surface. This non-destructive estimation of rock strength was conducted to address the challenge of relating core scale measurments to reservoir scaled analysis to improve hydraulic fracturing designs in unconventional source rocks.
Khan, Khaqan (Saudi Aramco) | Almarri, Misfer (Saudi Aramco) | Al-Qahtani, Adel (Saudi Aramco) | Syed, Shujath Ali (Baker Hughes, a GE Company) | Negara, Ardiansyah (Baker Hughes, a GE Company) | Jin, Guodong (Baker Hughes, a GE Company)
Rock mechanical properties are required as an input in many petroleum engineering applications, such as borehole stability analysis, hydraulic fracturing design, and sand production prediction. Their determination is commonly from various laboratory testing performed on subsurface rock samples. Due to the scarcity of reservoir samples and test cost, rock mechanical data are always very limited. Therefore, empirical correlations are very often used to estimate the mechanical properties from downhole logging measurements. Alternatively, the data-driven analytics techniques have been developed for predicting rock properties from other formation properties that can be determined directly from logs.
This paper presents a study of developing correlation equations and data-driven models that are used to predict the unconfined compressive strength (UCS) from logging data. Various rock mechanical tests including UCS, single- and multi-stage triaxial tests are performed on sandstone samples from three wells in one region. UCS values are obtained either from the UCS testing directly or from the Mohr-Coulomb failure analysis indirectly. Rock properties, such as mineralogy, porosity, grain and bulk density, ultrasonic wave velocities, are measured for each tested sample, which are used to build the correlations and data-driven analytical models for predicting UCS. Results shows that the empirical correlations are not universal and often cannot be used without some modifications, while the data-driven model is more generalized in application. In addition, data quality is very crucial for building correlations or predictive models.