Estimation of cement thermal properties through the three-phase model with application to geothermal wells

Geothermal energy has been used by mankind since ancient times. Given the limited geographical distribution of the most favorable resources, exploration efforts have more recently focused on unconventional geothermal systems targeting greater depths to reach sufficient temperatures. In these systems, geothermal well performance relies on efficient heat transfer between the working fluid, which is pumped from surface, and the underground rock. Most of the wells designed for such environments require that the casing strings used throughout the well construction process be cemented in place. The overall heat transfer around the wellbore may be optimized through accurate selection of cement recipes. This paper presents the application of a three-phase analytical model to estimate the cement thermal properties. The results show that cement recipes can be designed to enhance or minimize heat transfer around wellbore, extending the application of geothermal exploitation

Experimental Determination Of Oilfield Cement Properties And Their Influence On Well Integrity

Oilfield cements are a key element in well construction, during the operational phase of a well, and after its abandonment. Hydrocarbon, geothermal, gas storage and carbon sequestration wells make use of oilfield cements to seal the annular space between the casing and the formation, some of the most common cements being API Class G and H. With an increase in the world’s 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. In many scenarios, these properties are assumed constant with time, but an increasing number of authors have shown how properties change with time, temperature, and pressure. This work presents experiments run on Class G cement mixed with and without additives and cured at 25°C, 50°C, and 75°C, and constant pressure over a total of approximately two years. The influence of time, temperature, and additives on the performance of these cements is evaluated, and non-destructive testing methods are used to develop correlations to assess cement mechanical properties and to understand the development of porosity and its influence on cement performance. We applied NMR for the non-destructive evaluation of porosity, pore size distribution, cement water saturation, and curing of cement. We develop more accurate correlations as a function of temperature and for a variety of additives for the estimation of unconfined compressive strength through ultrasonic measurements

Estimation of plugging and abandonment costs based on different EU regulations with application to geothermal wells

This paper presents the cost of plugging and abandonment (P&A) of the Horstberg Z1 well and shows how the well history is considered in the P&A planning process together with existing European regulations. Three different design plans are presented, based on innovative ideas and best practices in the oil and gas industry. Horstberg Z1, located in Germany, was originally a gas well, converted to a scientific geothermal well to prove the single well concept. After serving as a geothermal well for several years, the well has been proposed for P&A. The three designs presented in this work fulfill the purposes of well P&A, and meet the BVOT requirements, which are standard requirements for wells in Lower Saxony, Germany. Following a financial analysis of the designs, it is noted that rig costs are the largest element of the total expenditure, contributing over 50% in the design plans considered. As the number of cement plugs and round trips increase per design plan, the implementation period also increases, which impacts on the total cost. Based on the investigations made in this project, a minimum of USD 1,275,500 is required for the plug and abandonment cost of the well, excluding the well site re-cultivation. A rigless solution for pulling heavy casing out of the wellbore, to avoid the use of a conventional workover rig, would significantly reduce the plug and abandonment cost of the Horstberg Z1 well

Estimation of Cement Thermal Properties through the Three-Phase Model with Application to Geothermal Wells

Geothermal energy has been used by mankind since ancient times. Given the limited geographical distribution of the most favorable resources, exploration efforts have more recently focused on unconventional geothermal systems targeting greater depths to reach sufficient temperatures. In these systems, geothermal well performance relies on efficient heat transfer between the working fluid, which is pumped from surface, and the underground rock. Most of the wells designed for such environments require that the casing strings used throughout the well construction process be cemented in place. The overall heat transfer around the wellbore may be optimized through accurate selection of cement recipes. This paper presents the application of a three-phase analytical model to estimate the cement thermal properties. The results show that cement recipes can be designed to enhance or minimize heat transfer around wellbore, extending the application of geothermal exploitation

Design Optimization of Geothermal Wells Using an Improved Overall Heat Transfer Coefficient

Geothermal wells have evolved in the past decade, moving from a concept copying oil and gas well construction and completion to modern and unconventional solutions applied to local conditions. In general, geothermal wells are optimized based on their flow rate output. The optimization results are visible as the modern geothermal wells have large casing sizes, no production tubing down to the reservoir, and so on. When doublets are required, drilling them from the same location can reduce costs. However, this approach must first answer the following questions: which well should be the injector, the producer, and shall these wells have the same design? In previously published papers, it was presented that well cementing and the drilling process affects the heat transfer along the wellbore. Using appropriate cement composition and by controlling the filter cake thickness and mud filtrate invasion, the heat transfer can be reduced or enhanced. These finding are used in this paper to improve well design with the focus on maximizing the heat output. The paper herein is proposing a theoretical discussion about wellbore overall heat transfer coefficient with a focus on geothermal well optimization based on the downhole heat exchange, leading to an improved geothermal well design. Furthermore a new well construction will be shown based on the proposed optomization method

Design Optimization of Geothermal Wells Using an Improved Overall Heat Transfer Coefficient

Geothermal wells have evolved in the past decade, moving from a concept copying oil and gas well construction and completion to modern and unconventional solutions applied to local conditions. In general, geothermal wells are optimized based on their flow rate output. The optimization results are visible as the modern geothermal wells have large casing sizes, no production tubing down to the reservoir, and so on. When doublets are required, drilling them from the same location can reduce costs. However, this approach must first answer the following questions: which well should be the injector, the producer, and shall these wells have the same design? In previously published papers, it was presented that well cementing and the drilling process affects the heat transfer along the wellbore. Using appropriate cement composition and by controlling the filter cake thickness and mud filtrate invasion, the heat transfer can be reduced or enhanced. These finding are used in this paper to improve well design with the focus on maximizing the heat output. The paper herein is proposing a theoretical discussion about wellbore overall heat transfer coefficient with a focus on geothermal well optimization based on the downhole heat exchange, leading to an improved geothermal well design. Furthermore a new well construction will be shown based on the proposed optomization method

Influence of Cement Thermal Properties on Wellbore Heat Exchange

Cementing geothermal wells is not an easy task. Besides the high temperature environment that affects the curing time of the cement, the cement sheath behind the casing, which is used primarily to protect and hydraulically seal the wellbore, influences the heat exchange from the wellbore to the formation and vice-versa. Depending on the geothermal operation, high or low heat exchanges might be preferred. For example, operators aim for a high and constant heat exchange in the lower part of a geothermal well and will try to insulate the upper part of the same well in order to minimize heat and efficiency losses accordingly. This study focuses on the ability of different cements’ thermal conductivities to influence the heat exchange in various wellbore segments under assumed static conditions. Sensitivities indicate which parameters play crucial roles while modeling the heat transfer in high temperature boreholes. Via Monte Carlo simulation, the relative importance of diverse thermal and geometrical parameters is considered when calculating the rock formation/wellbore heat exchange. The modeling highlighted the presence of a ‘working window' in which the variation of the thermal or geometrical parameters can strongly influence the borehole-formation heat exchange. Finally, the paper provides the optimum cement thermal properties required to properly cement a geothermal well and so minimize/enhance heat exchange to/from the wellbore

The Influence of Remedial Cementing on Thermal Well Design with Applications to Wellbore Integrity

The safe operation of geothermal wells is usually achieved with a two-barrier philosophy, similar to that used in oil and gas wells, where production tubing and/or casing constitute the primary barrier to production loads, with the wellbore cement representing the second integrity-assuring element. Well cement plays an important role in providing well integrity, yet also influences wellbore heat transfer, but its mechanical and thermal properties can vary significantly depending on the mixing method employed, the slurry composition, curing conditions, and human factors. Mechanical strength still represents the major criterion for cement selection, although thermal properties of cements represent an important parameter in the efficient design of geothermal wells. This paper focuses on the effects of remedial cementing on wellbore integrity and heat transfer, considering how the chosen method of remediation may result in different degrees of efficiency. From experiments designed to reproduce such remedial actions, cement properties are assessed and used to quantify the effect of these procedures on the overall heat transfer in the wellbore, in regards to both thermal stresses and heat exchange. Testing shows that cement mixing with annular fluid during the remedial phase will lead to changes in cement heat transfer properties and may increase the localized wellbore stresses

Effect of Mud Filter Cake on Heat Transfer in Geothermal Wellbores

The drilling of deep wells requires the use of performance muds that can support and protect the wellbore during the process of ‘making hole’. Before and during cement jobs, the goal is to remove as much as possible of the existing filter cake (a by-product of the drilling mud) in order to enhance the quality of the annular cement sheath. Yet, experiments have shown that often a thin filter cake stubbornly remains between cement and formation. This is a cause for concern as the effectiveness of geothermal wells relies on maximizing the heat mining of the reservoir, and optimizing the heat transfer process is a major consideration. The effect of temperature on drilling fluids has undergone thorough investigations which show that high temperatures cause a degradation of rheological properties, as well as increasing fluid loss. In heat transfer estimations for geothermal well design, the required inputs relating to the drilling mud properties will be flawed, unless the filter cake is taken into consideration. This paper focuses on laboratory investigations of filter cake thermal capacity and conduction, followed by a theoretical estimation of the effect of filter cake on the wellbore heat transfer process. The results show that conventional water based muds can impair the heat transfer from and to the wellbore. Two mud samples were tested and it was observed that their thermal conductivities increased with temperature. It was noted that adding Barite to the drilling mud increased the thermal diffusivity of the filter cake, but it was found to be lower than that of the drilling fluid that produced it

Effect of Mud Filter Cake on Heat Transfer in Geothermal Wellbores

The drilling of deep wells requires the use of performance muds that can support and protect the wellbore during the process of ‘making hole’. Before and during cement jobs, the goal is to remove as much as possible of the existing filter cake (a by-product of the drilling mud) in order to enhance the quality of the annular cement sheath. Yet, experiments have shown that often a thin filter cake stubbornly remains between cement and formation. This is a cause for concern as the effectiveness of geothermal wells relies on maximizing the heat mining of the reservoir, and optimizing the heat transfer process is a major consideration. The effect of temperature on drilling fluids has undergone thorough investigations which show that high temperatures cause a degradation of rheological properties, as well as increasing fluid loss. In heat transfer estimations for geothermal well design, the required inputs relating to the drilling mud properties will be flawed, unless the filter cake is taken into consideration. This paper focuses on laboratory investigations of filter cake thermal capacity and conduction, followed by a theoretical estimation of the effect of filter cake on the wellbore heat transfer process. The results show that conventional water based muds can impair the heat transfer from and to the wellbore. Two mud samples were tested and it was observed that their thermal conductivities increased with temperature. It was noted that adding Barite to the drilling mud increased the thermal diffusivity of the filter cake, but it was found to be lower than that of the drilling fluid that produced it