Drilling optimization requires detailed engineering in all aspects of well planning, drilling implementation, and post-run evaluation Effective well planning optimizes the boundaries, constraints, learning, nonproductive time, and limits and uses new technologies as well as tried and true methods. Use of decision support packages, which document the reasoning behind the decision-making, is key to shared learning and continuous improvement processes. It is critical to anticipate potential difficulties, to understand their consequences, and to be prepared with
contingency plans. Post-run evaluation is required to capture learning.
Many of the processes used are the same as used during the well planning phase, but are conducted using new data from the recent drilling events. Depending on the phase of planning and whether you are the operator
or a service provider, some constraints will be out of your control to alter or influence (e.g., casing point selection, casing sizes, mud weights, mud types, directional plan, drilling approach such as BHA types or new technology
use). There is significant value inbeing able to identify alternate possibilities for improvement over current methods, but well planning must consider future availability of products and services for possible well interventions.
When presented properly to the groups affected by the change, it is possible to learn why it is not feasible or to alter the plan to cause improvement. Engineers must understand and identify the correct applications for technologies to reduce costs and increase effectiveness.Acorrect application understands the tradeoffs of risk versus rewardandcosts versus benefits.
Boundaries Boundaries are related to the “rules of the game” established by the company or companies involved. Boundaries are criteria established by management as “required outcomes or processes” and may relate to
behaviors, costs, time, safety, and production targets.
Constraints Constraints during drilling may be preplanned trip points for logs, cores, casing, and BHA or bit changes. Equipment, information, human resource knowledge, skills and availability, mud changeover, and dropping balls for downhole tools are examples of constraints on the plan and its implementation.
The Learning Curve Optimization’s progress can be tracked using learning curves that chart the performance measures deemed most effective for the situation and then applying this knowledge to subsequent wells.
Learning curves provide a graphic approach to displaying the outcomes. Incremental learning produces an exponential curve slope. Step changes may be caused by radically new approaches or unexpected trouble. With
understanding and planning, the step change will more likely be in a positive direction, imparting huge savings for this and future wells. The curve slope defines the optimization rate. The learning curve can be used to demonstrate the overall big picture or a small component that affects the overall outcome. In either case, the curve measures the rate of change of the parameter you choose, typically the “performance measures” established by you and your team. Each performance measure is typically plotted against time, perhaps the chronological order of wells drilled as shown in figure below:
Cost Estimating Oneof the mostcommonand critical requests of drilling engineers is to provide accurate cost estimates, or authority for expenditures (AFEs). The key is to use a systematic and repeatable approach that takes
into account all aspects of the client’s objectives. These objectives must be clearly defined throughout the organization before beginning the optimization and estimating process. Accurate estimating is essential to maximizing a company’s resources. Overestimating a project’s cost can tie up capital that could be used elsewhere, and underestimating can create budget shortfalls affecting overall economics.
Integrated Software Packages With the complexity of today’s wells, it is advantageous to use integrated software packages to help design all aspects of the well. Examples of these programs include
• Casing design
• Torque and drag
• Directional planning
• Well control
Decision Support Packages Decision support packages document the reasoning behind the decisions that are made, allowing other people to understand the basis for the decisions. When future well requirements change, a decision trail is available that easily identifies when new choices may be needed and beneficial.
Performance Measures Common drilling optimization performance measures are cost per foot of hole drilled, cost per foot per casing interval, trouble time, trouble cost, and AFEs versus actual costs.
Systems Approach Drilling requires the use of many separate pieces of equipment, but they must function as one system. The borehole should be included in the system thinking. The benefit is time reduction, safety improvement, and production increases as the result of less nonproductive time and faster drilling. For example, when an expected average rate of penetration (ROP) and a maximum instantaneous ROP have been identified, it is possible to ensure that the tools and borehole will be able to support that as a plan. Bit capabilities must be matched to the rpm, life, and formation. Downhole motors must provide the desired rpm and power at the flow rate being programmed. Pumps must be able to provide the flow rate and pressure as planned.
Nonproductive Time Preventing trouble events is paramount to achieving cost control and is arguably the most important key to drilling a cost-effective, safe well. Troubles are “flat line” time, a terminology emanating from the days versus depth curve when zero depth is being accomplished for a period of days, creating a horizontal line on the graph. Primary problems invariably cause more serious associated problems. For example, surge pressures can cause lost circulation, which is the most common cause of blowouts. Excessive mud weight can cause differential sticking, stuck pipe, loss of hole, and sidetracking. Wellbore instability can cause catastrophic loss of entire hole sections. Key seating and pipe washouts can cause stuck pipe and a fishing job.
When a trouble event leads to a fishing job, “fishing economics” should be performed. This can help eliminate emotional decisions that lead to overspending. Several factors should be taken into account when determining
whether to continue fishing or whether to start in the first place.
The most important of these are replacement or lost-in-hole cost of tools and equipment, historical success rates (if known), and spread rate cost of daily operations. These can be used to determine a risk-weighted value of
fishing versus the option to sidetrack.
Operational inefficiencies are situations for which better planning and implementation could have saved timeandmoney. Sayings such as“makin’ hole” and “turnin’ to the right” are heard regularly in the drilling business.
These phases relate the concept of maximizing progress. Inefficiencies which hinder progress include
• Poor communications
• No contingency plans and “waiting on orders” (WOO)
• Tool failure
• Improper WOB and rpm (magnitude and consistency)
• Mud properties that may unnecessarily reduce ROP (spurt loss, water loss and drilled solids)
• Surface pump capacities, pressure and rate (suboptimum liner selection and too small pumps, pipe, drill collars)
• Poor matching of BHA components (hydraulics, life, rpm, and data acquisition rates)
• Survey time
Limits Each well to be drilled must have a plan. The plan is a baseline expectation for performance (e.g., rotating hours, number of trips, tangibles cost). The baseline can be taken from the learning curves of the best experience that characterizes the well to be drilled. The baseline may be a widely varying estimate for an exploration well or a highly refined measure in a developed field. Optimization requires identifying and improving on the limits that play the largest role in reducing progress for the well being planned. Common limits include
1. Hole Size. Hole size in the range of 7 7/8 – 8 1/2 in. is commonly agreed to be the most efficient and cost-effective hole size to drill, considering numerous criteria, including hole cleaning, rock volume drilled, downhole tool life, bit life, cuttings handling, and drill string handling. Actual hole sizes drilled are typically determined by the size of production tubing required, the required number of casing points, contingency strings, and standard casing decision trees. Company standardization programs for casing, tubing, and bits may limit available choices.
2. Bit Life. Measures of bit life vary depending on bit type and application. Roller cones in soft to medium-soft rock often use KREVs (i.e., total revolutions, stated in thousands of revolutions). This measure fails to consider the effect ofWOBon bearing wear, but soft formations typically use medium to high rpm and low WOB; therefore, this measure has become most common. Roller cones in medium to hard rock often use a multiplication of WOB and total revolutions, referred to as the WR or WN number, depending on bit vender. Roller cone bits smaller than 7 7/8 in. suffer significant reduction in bearing life, tooth life, tooth size, and ROP. PDC bits, impregnated bits, natural diamond bits, and TSP bits typically measure in terms if bit hours and KREVs. Life of all bits is severely reduced by vibration. Erosion can wear bit teeth or the bit face that holds the cutters, effectively reducing bit life.
3. Hole Cleaning. Annular velocity (AV) rules of thumb have been used to suggest hole-cleaning capacity, but each of several factors, including mud properties, rock properties, hole angle, and drill string rotation, must be considered. Directional drilling with steerable systems require “sliding” (not rotating) the drill string during the orienting stage; hole cleaning can suffer drastically at hole angles greater than 50. Hole cleaning in large-diameter holes, even if vertical, is difficult merely because of the fast drilling formations and commonly low AV.
4. Rock Properties. It is fundamental to understand formation type, hardness, and characteristics as they relate to drilling and production. From a drilling perspective, breaking down and transporting rock (i.e., hole cleaning) is required. Drilling mechanics must be matched to the rock mechanics. Bit companies can be supplied with electric logs and associated data so that drill bit types and operating parameters can be recommended that will match the rock mechanics. Facilitating maximum production capacity is given a higher priority through the production zones. This means drilling gage holes,minimizing formation damage (e.g., clean mud, less exposure time), and facilitating effective cement jobs.
5. Weight on Bit. WOB must be sufficient to overcome the rock strength, but excessive WOB reduces life through increased bit cutting structure and bearing wear rate (for roller cone bits). WOB can be expressed in terms of weight per inch of bit diameter. The actual range used depends on the “family” of bit selected and, to some extent, the rpm used. Families are defined as natural diamond, PDC, TSP (thermally stable polycrystalline), impregnated, mill tooth, and insert.
6. Revolutions per Minute (rpm). Certain ranges of rpm have proved to be prudent for bits, tools, drill strings, and the borehole. Faster rpm normally increases ROP, but life of the product or downhole assembly may be severely reduced if rpm is arbitrarily increased too high. A too-low rpm can yield slower than effective ROP and may provide insufficient hole cleaning and hole pack off, especially in high-angle wells.
7. Equivalent Circulating Density (ECD). ECDs become critical when drilling in a soft formation environment where the fracture gradient is not much larger than the pore pressure. Controlling ROP, reducing pumping flow rate, drill pipe OD, and connection OD may all be considered or needed to safely drill the interval.
8. Hydraulic System. The rig equipment (e.g., pumps, liners, engines or motors, drill string, BHA) may be a given. In this case, optimizing the drilling plan based on its available capabilities will be required.
However, if you can demonstrate or predict an improved outcome that would justify any incremental costs, then you will have accomplished additional optimization. The pumps cannot provide their rated horsepower
if the engines providing power to the pumps possess inadequate mechanical horsepower. Engines must be down rated for efficiency.
Changing pump liners is a simple cost-effective way to optimize the hydraulic system. Optimization involves several products and services and the personnel representatives.This increases the difficulty to achieve an optimized parameter selection that is best as a system.
Positive step changes reflected in the learning curve are often the result of effective implementation of new technologies:
1. Underbalanced Drilling. UBD is implemented predominantly to maximize the production capacity variable of the well’s optimization by minimizing formation damage during the drilling process. Operationally, the pressure of the borehole fluid column is reduced to less than the pressure in the ZOI. ROP is also substantially increased. Often,
coiled tubing is used to reduce the tripping and connection time and mitigate safety issues of “snubbing” joints of pipe.
2. Surface Stack Blowout Preventer (BOP). The use of a surface stack BOP configurations in floating drilling is performed by suspending the BOP stack above the waterline and using high-pressure risers (typically 13 3/8 in. casing) as a conduit to the sea floor. This method, generally used in benign and moderate environments, has saved considerable time and money in water depths to 6,000 ft.
3. Expandable Drilling Liners. EDLs can be used for several situations. The casing plan may startwith a smaller diameter than usual, while finishing in the production zone as a large, or larger, final casing diameter. Future advances may allow setting numerous casing strings in succession, all of the exact same internal diameter. The potential as a step change technology for optimizing drilling costs and mitigating risks is phenomenal.
4. Rig Instrumentation. The efficient and effective application of weight to the bit and the control of downhole vibration play a key role in drilling efficiency. Excessive WOB applied can cause axial vibration, causing destructive torsional vibrations. Casing handling systems and top drives are effective tools.
5. Real-Time Drilling Parameter Optimization. Downhole and surface vibration detection equipment allows for immediatemitigation. Knowing actual downhole WOB can provide the necessary information to perform improved drill-off tests .
6. Bit Selection Processes. Most bit venders are able to use the electric log data (Sonic,GammaRay, Resistivity as aminimum)and associated offset information to improve the selection of bit cutting structures. Formation
type, hardness, and characteristics are evaluated and matched to the application needs as an optimization process.