WP2 - Well Drilling

[Months: 4-30]


The objective of this work-package is to deep the Venelle 2 well down to supercritical conditions. This includes:

  • General preparation activities
  • Special Cement
  • Special Casing
  • Special Rock Bit and other BHA
  • Drilling fluid & cooling
  • Real Time Well Control
  • Execution of drilling
  • Logging, coring and fluid sampling
  • Production test
  • HSE: Health, Safety and Environmental protection measures

Task 2.1 General preparation activities (EGP)

  • Planning of the drilling phase: water supply, pad preparation, logistics, permitting, moving of drilling rig.

Task 2.2 Well Material (EGP)

  • Cement: The main difficulties are related to the high temperatures and the corrosive fluid environment. The specification of a suitable Bottom Hole Circulating Temperature (BHCT) of the slurry and a specific cementing job procedure are required. A cement type capable of maintaining the appropriate characteristics at high temperature is needed. One possible solution is to test Halliburton Thermalock cement as it is suitable for a corrosive CO2 and acid environment, thereby protecting pipes and casing, and also works at high temperatures (up to 370°C). It is a non-Portland cement, based on calcium phosphate. A set of laboratory measurements are available up to 370°C, but additional laboratory trials will be planned as new applied research for this project. The trials will evaluate resistance under repeated cycles of heating/cooling and different pH conditions. This cement has been used up to 250°C (Indonesia), and its resistance to higher temperatures and in a real well will be tested in the supplier’s laboratory.
  • Casing: The criticalities are related to high temperature, high pressure and corrosivity, which are assumed to be present with super-critical fluids. The type of casing that can withstand the expected temperature and corrosiveness conditions needs to be identified as well as the wellhead components able to withstand high pressures, despite the de-rating caused by temperature. An evaluation of the possible casing material and select the most appropriate corrosion resistant alloy (CRA) for our environment will be performed, based on simulation of the expected drilling conditions and different possible commercial solution.
  • BHA: Within the BHA components there are: rock bits, stabilizers, and drill pipes/ drill collars. The main problem caused by the harsh environment is linked to the rock bits, as the high temperature affects the seals of the bearings. The presence of a highly corrosive chemical environment also needs to be evaluated.
  • Drilling fluid: Drilling fluid (water or mud) is considered as a critical element due to the critical pressure and temperature conditions expected. Constant heat removal from the bottom hole to the surface is required to cool the wellbore down to acceptable temperatures (<250°C) while drilling. At the same time, boiling of the fluid at atmospheric pressure must be avoided. Such high pressures can also dictate the need for heavier fluids in order to ensure the primary control of the well. While circulating drilling mud at high flow rates can cool the well, the frictional pressure losses incurred by the high flow rate may take the wellbore pressure outside the pressure window allowed by the surrounding formation. This is further complicated by the fact that frictional pressure loss itself is sensitive to temperature. Hydraulic simulation of these effects, considering the heat exchange with the formation, will be used to determine the minimum and maximum flow rates necessary to keep conditions sub-critical while also keeping the well pressure within the allowed ranges. One possible experimental solution will be to test novel synthetic fluid-loss polymers, for making water-based mud systems stable for high temperature, high pressure drilling, like Thermadrill from Halliburton. These systems are able to maintain their properties up to 250°C, thus avoiding stuck conditions while drilling due to mud solidification.

Task 2.3 Real Time well control (EGP, SINTEF PR)

  • The super-critical phase poses a challenge to drilling operations. Knowing when and where the phase occurs and whether it can be avoided would enable deep geothermal wells to be drilled faster, with less chance of failure and at less cost. Our solution is to combine expertise from the oil and gas industry with fundamental research on super-critical transitions. An oil and gas flow model provided by one partner will be used. This is a multi-phase real-time drilling model underpinning the commercially available eDrilling software. New simulation algorithms will be combined with the commercially available LedaFlow simulator for single and multiphase flows. A simulator originally developed for use in the oil and gas industry will be used to predict the transport of heat and fluids in a super-critical well, enabling the drilling crew to predict downhole pressures even when the well conditions go beyond the capacity of commercially available wellbore models. The simulator is based on a 1D multiphase model with momentum, mass and energy equations for each phase. Extensions will be made to model the transitional region where the fluids enter the super-critical state. The model will be validated on the Venelle 2 well. It will also provide simulations of the well in full production, by interfacing the newly-developed model with the LedaFlow simulation tool.

Task 2.4 Execution of Drilling (EGP, SINTEF PR)

  • A specific well design will be defined on the basis of the current Venelle 2 situation and the expected super-critical conditions. The design will ensure safe drilling and testing, and minimize any environmental impact. Procedures will be developed to ensure the well is monitored both in normal operations and during contingencies. The execution of the drilling will be done by EGP crew, applying the best state of the art of the knowledge and working experience on such subject. Special attention will be paid on the HSE aspects, with specific measure to minimize the risk of accidents, blow out, dispersion into the environment of geothermal fluids and gases.

Task 2.5 Logging, Coring, Fluid Sampling (EGP)

  • Before and during drilling a standard set of geophysical logs will be performed, as well as sampling of cuttings, coring in different depth and fluid sampling, for performing a series of scientific activities, focussing both indications and information for the drilling phase and characterization of the depth environment.
    Some specific standard logging tools for the Neutron, Density and FLeXSM measurements will utilize a radioactive source (usually employed in the standard drilling activities, both for oil and gas and geothermal), which will be handled under the standard radiation hazard safety conditions by trained and qualified operators of the service company, and only in case of minimum risk for the operative situations of the well, in order to avoid any exposition to operators and diffusion of radioactive contamination into the environment.
    During the drilling phase a relevant series of deep fluid samples will be taken for laboratory analysis; the most important fluid sample will arise from the production test after the well completion or from deep relevant productive fractures. A detailed sampling procedure will be agreed between partners, in order to achieve the best suitable quality of the samples. A specific attention will be dedicated to gas sampling during drilling from the mud degassing system, as provided by the mud logging service, which can provide an offline recording of the deep fluid ascent during penetration of the well in depth, monitoring noble gasses isotopes, with special focus on He, identifying approximately ten gas samples for detailed laboratory analysis.
    Moreover, samples from mud, after a specific treatment will be analysed in laboratory for chemical analyses of selected parameters (Cl, B, Li), in addition to the standard determination made on drilling site.
    The plan is to take at least one or two core sections from the well at a safe depth. This will offer the opportunity to perform several petrophysical laboratory analyses on core samples so as to define important physical characteristics of the reservoir host rocks, such as the thermal, electrical, hydraulic and geomechanical properties. Several existing rock samples from nearby wells in the metamorphic basement could be analyzed before drilling, for acquisition of a reference dataset. The possibility of performing a Sidewall Coring Technology run will be explored, accordingly to the drilling conditions and its constraints, for a collection of at lease 10 cores from the wall at a selected depth, after the completion of the well.

Task 2.6 Production test (EGP)

  • At the end of the drilling phase, according to the well conditions, a production test of the deep fluid will be performed, aimed to characterize the performances of the supercritical environment that assesses the true reservoir potential at full scale under dynamic conditions and the chemical and physical characterization of its fluids.