A proprietary cementing-job simulation software has been upgraded with a losses model. Using the losses model will enable the design engineer to define loss zones in the wellbore. For each identified loss zone, a percentage of losses can be provided. Loss-input and other well and fluid data allow a detailed dynamic simulation. Use of novel loss-zone simulation during cementing-job simulations enables the design team to discuss potential outcomes of cementing operations while experiencing losses.
Lost Circulation While Cementing
Although losses can occur during all phases of the life of the well, losses during primary cementing can have a major effect on well safety and economics. If losses cannot be cured ahead of the cementing job or occur unexpectedly during it, they can potentially jeopardize the outcome of the job, require remedial work, and lead to the loss of the interval or, in the worst case, the entire well. The eventual outcome of the cementing job will depend on several factors: loss rate or severity, the depth of the loss zone compared with the proposed tops of fluids, when the losses occur during the cement job, and prevention and mitigation measures taken during the cement job.
The most obvious result of losses during the cement job is lower top of cement (TOC) in the annulus. Experiencing partial to severe losses can reduce the TOC from reaching potential regulatory requirements. It can also result in partial or no isolation across planned production zones or potential flow zones.
Losses also affect mud removal in the annulus and the zonal isolation of the cement sheath. If losses occur, the spacer designed to remove the drilling fluid can be lost to the formation, resulting in increased contamination of the cement slurry. Additionally, fluid velocities will be affected by losses and can result in poor mud-removal isolation across zones of interest. Poor zonal isolation can result in lower production rates, unwanted water production, and casing exposure to corrosive formation fluids.
Loss Mitigation and Prevention
The ability to simulate losses and predict results by considering the fluid contamination and TOC can be used in the decision-making process to either cure the losses before the cementing job or implement other mitigation measures, such as adding lost-circulation material to the spacer or slurry, adjusting planned volumes, or adjusting pump rates. Alternatively, if the predicted outcome is acceptable for the well design, the decision can be made to proceed with the cementing job with caution.
Even if losses are not present before the job and are not expected based on offset wells, they sometimes can appear during job execution. In case of unexpected losses during the cementing job, the ability to simulate the effect using actual cementing-job data immediately afterward can assist with decision-making after the job is complete. For example, it can warrant that an additional cement-evaluation log be run to confirm that cementing-job objectives have been achieved or if a remedial job is required to proceed safely with drilling operations.
Lost-Circulation Simulator
When designing cementing jobs, a critical step consists in ensuring proper control of downhole pressure to keep it within the pore and fracture pressure window agreed upon with the drilling team. To that end, hydraulic simulations are performed to evaluate the dynamic pressure within the well during the cementing operation. During cementing-job execution, several different fluids are pumped into the well. As a result, the drilling fluid originally present within the well is partially or fully displaced out of it. Therefore, the simulation must keep track of different fluids and their properties. The hydraulic simulation must be coupled with a dynamic temperature simulation so that relevant fluid properties are determined throughout the simulated domain. The equations that govern pressure within the well are obtained by simplifying the well-known Navier-Stokes equations under several assumptions, the main one being that the flow is one-dimensional.
The hydraulic simulator solves important equations identified in the complete paper numerically to obtain the pressure, velocity, and temperature in the flow path dynamically throughout the cementing job.
The inputs needed to run the simulation include not only the usual hydraulic-simulation inputs but also inputs specific to losses. These include the loss-zone top and bottom measured depths and a loss schedule where the user defines the expected loss rate vs. time as a percentage of pump rate. This percentage can be defined as 100%, this value corresponding to total losses. Instead of a predefined schedule of flow rates, the simulator instead can use the pump-rate time channel acquired during job execution as input. If available, the simulator also will take advantage of the time channel providing the density of the fluids pumped into the well. When used in this mode, the simulator can be used to match the acquired pump pressure. Achieving a pressure match is considered a good indication that the simulation outputs reflect what happened within the well.
The outputs from the simulation, first and foremost, are the usual hydraulic-simulation outputs: pressure vs. depth at any time during the simulation, temperature vs. depth at any time during the simulation, positions of the fluids in the wellbore, bottomhole circulating pressure, and bottomhole circulating temperature. The consequences of the losses will be seen not only for the positions of the fluids but also for the velocity of the fluids in the annulus above the loss zone, which will be less than in a scenario without losses. In turn, this will be reflected in the equivalent-circulating-density (ECD) outputs. Similarly, the lower velocity will lead to less heat exchange between the annulus and the formation on one side and the annulus and the pipe on the other side, therefore affecting the bottomhole circulating temperature and the temperature profiles. Finally, specific outputs also are provided: loss rate at the loss zone vs. time and cumulative volume lost vs. time.
Particular care was given to computation performance when developing the simulator, such that simulation runs are completed in a matter of seconds, thereby enabling multiple scenarios to be explored efficiently.
In the complete paper, case studies are provided wherein the loss-circulation model was used during the design phase as well as during the job-evaluation phase. One of these case studies is included in this synopsis.
Case Study: Unexpected Losses in 7-in. Casing in Western Desert
This case study analyzes an incident of an unforeseen lost circulation event during a 7-in. liner cementing operation in a Western Desert exploration well. Before cementing, a thorough discussion with the operator highlighted the absence of detailed fracture-gradient data, necessitating its estimation based on distant-offset‑well information.
Relying on this estimated data, the operator determined a fracture gradient of 15 lbm/gal. The subsequent cementing-job design, validated through successful well-security simulations, adhered to these parameters, showing that the maximum annular pressure ECD was 14.52 lbm/gal at total depth during cement placement.
Before the cementing job, minimal losses of approximately 10 bbl/hr were reported during prejob circulation. During cementing-job execution, however, a sudden and unexpected increase in losses occurred while pumping the displacement fluid with total losses and no fluid returns to surface. This resulted in the TOC not reaching the planned depth at the top of the liner.
To estimate the actual TOC and determine the need for remedial actions, a lost circulation zone was input during a post-job pressure-matching simulation. The simulation, incorporating an expected loss zone at 12,500 ft, closely matched the actual pumping pressures observed during the cementing job. This simulation confirmed that the TOC reached a depth sufficient to cover the pay zone, eliminating the need for a remedial squeeze job.
Lost circulation influences wellbore temperatures. The change in bottomhole circulating temperature (BHCT) and the wellbore-temperature profile are dependent on the loss rate as well as fluid, wellbore, and formation parameters. As the overall wellbore temperature reduces owing to losses, lost circulation can result in longer cement-setting times. Using the simulator to account for losses, an accurate wellbore temperature profile and BHCT can be provided. This new temperature profile can be used to repeat critical slurry testing and determine rig operations such as delaying tagging or drilling out cement or delaying log evaluations until the cement has built sufficient compressive strength.
A subsequent cement-bond evaluation log confirmed the TOC at 12,500 ft, aligning with the post-job simulated top of losses determined by the simulations. Based on this confirmation, no remedial squeeze cementing operations were deemed necessary, as shown in Fig. 1 above.
Conclusions
Lost circulation can jeopardize cementing-job objectives and can have significant consequences for the well in case zonal-isolation requirements are not met. In the ideal situation, losses should be cured before the start of the cementing job. However, that is not always feasible; sometimes losses occur unexpectedly.
The ability to accurately simulate the outcome of a cementing job under a lost-circulation situation, either during the prejob planning phase or during post-job cement evaluation, can help the drilling engineer and cementing engineer make educated assumptions on the risks and outcomes of the cementing job. Being able to simulate accordingly will allow for adjustments to the cementing-job design or well design, as well as improve rig operations with better planning ahead for remedial steps such as scheduling for wireline-evaluation logs or remedial cementing jobs.
Download complete technical paper here. Available until 30 June 2025.
This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 223263, “Simulating Lost Circulation in a Primary Cementing-Job Design,” by Martijn Bogaerts, SPE, Nicolas Flamant, SPE, and Ahmed Abdulaal, SPE, SLB, et al. The paper has not been peer reviewed.
