Building a drilling programme from offset data

A drilling programme is a set of decisions the rig will live with for a month: where each casing string goes, what mud holds the hole, how long each step takes and what it costs. The offset wells have already made most of those decisions once, some well and some badly. This article builds a programme for a vertical development well from four offsets, step by step, and shows the checks that catch a draft's mistakes before they reach the rig.

What the programme has to settle

Every programme answers the same questions in the same order. The pressure window and the trouble zones set the casing points. The casing points set the hole sizes, and each hole size must pass the drift of the string above it. The window in each section sets the mud weight. The sequence of steps, each with a duration, gives the days to TD and to rig release. Days times the spread rate, plus the casing, cement, bits and logs, gives the cost. The hazards by depth, with a contingency for the ones that are likely and expensive, close it.

Offset wells feed every one of those steps. Their leak-off tests and pressure data shape the window. Their daily reports say where they lost returns, where the hole was tight and where the pipe stuck. Their time logs say how long each step really took. A programme written without them repeats their problems; one written from them can plan around those problems.

The workflow

1. Collect the offsets on the same structure and the same casing scheme: tops, casing points, mud weights, shoe tests, NPT events with depth and hours, and the time for every step.
2. Build the window. Pore pressure from the offsets' pressure tests, fracture pressure from their leak-off tests, and the loss zones from their reports. The usable window is the pore pressure plus a trip margin to the fracture pressure less a surge margin.
3. Place the casing seats where the window has to change, below each loss zone and above each zone that needs more weight than the hole above can take.
4. Choose the strings and holes so each bit passes the drift of the casing it runs through.
5. Time each step from the offsets. The median of the offsets is a fair plan; the fastest offset at each step is the technical limit to aim at.
6. Cost each section: its days times the spread rate, plus its one-off lines.
7. Register the hazards by depth, score them, and attach a contingency, with its days and cost, to each one worth one.
8. Check the whole programme: mud weight against the window and the shoe tests, drift, liner lap, mud coverage from spud to TD, targets and the step depths.

The example well

The example is a vertical oil producer set in typical onshore Kuwait conditions, to the Mauddud limestone at 2,240–2,330 m. The column above it is the familiar one: the unconsolidated Kuwait Group at surface, the Dammam limestone, the Rus anhydrite and the Radhuma dolomite aquifer, then the Tayarat, Hartha, Sadi and Tanuma, the Mishrif and Rumaila carbonates, and the Ahmadi shale and Wara sand directly above the target. Four offsets on the same structure were drilled with the same casing scheme.

The 9⅝″ seat is the decision that matters most. The Ahmadi needs about 1.25 sg to stay in gauge, and the depleted Mishrif above it loses mud well below that. Setting the 9⅝″ shoe between them lets the 12¼″ hole stay at 1.15 sg and the 8½″ hole go to 1.25 sg. The cost of the choice is the Wara: at 1.25 sg the depleted sand sees a large overbalance, which is why the Wara carries the highest-scoring hazard in the register.

Hand checks

1. Drift. API casing drift is the inside diameter less 5/32″ for 9⅝″ to 13⅜″ casing. 9⅝″ 47 lb/ft has an ID of 8.681″, so its drift is 8.525″ and an 8½″ bit passes. 9⅝″ 53.5 lb/ft has an ID of 8.535″ and a drift of 8.379″: an 8½″ bit does not pass. The 13⅜″ 68 lb/ft string has a drift of 12.259″, which a 12¼″ bit clears by 0.009″.
2. Overbalance on the Wara. Mud at 1.25 sg against a pore pressure of 1.03 sg is 0.22 sg over. At 2,200 m that is 0.22 × 1.4223 × 2,200 ≈ 690 psi pressing the pipe into a permeable, depleted sand.
3. A step from the offsets. The 16″ hole took 3.0, 3.5, 4.5 and 6.5 days. The median is (3.5 + 4.5) / 2 = 4.0 days; the fastest, 3.0 days, is the technical limit for that step.
4. A section's cost. The 12¼″ section is 7.5 days. At a spread of 45,000 USD a day (rig 28,000, services 17,000) that is 337,500; with 400,000 of casing, cement, bits and a tubing spool, the section costs 737,500 USD.

Checking the draft

The first draft of this programme copied two things from offsets without checking them: the heavier 9⅝″ string from a deeper well's programme, and the 8½″ mud weight of 1.32 sg from the offset that had tight hole in the Ahmadi. It also set the liner top 20 m inside the 9⅝″ shoe. DrillIQ's checks, run on the draft against the pressure window and the shoe tests, gave two errors and a warning.

The final programme passes every engineering check: each mud weight sits inside its window (1.06 sg in 1.03–1.39, 1.08 in 1.04–1.11, 1.15 in 1.07–1.42, 1.25 in 1.13–1.58), each bit passes the drift above it, the mud programme covers 30 m to 2,330 m with no gap, and every drilling step ends at a shoe or TD. Kick tolerance is listed as not checked, because no company minimum was entered; that is a standard to set, not a pass.

The checks also suggest hazards the planners had not written down. The window in the 16″ section is only 0.07 sg wide at its narrowest, between a 1.04 sg floor and a 1.11 sg ceiling, so the engine flags it: losses on one side, a kick on the other. Drilling blind with water when returns are lost lowers the hydrostatic head, so the right response is a kick hazard in the register and a flow check on every connection in that section.

Time

Taking each step at the median of the offsets gives 29.0 days from the rig move to release, with TD on day 23. The offsets themselves took 26.0, 26.5, 29.75 and 38.0 days. The sum of the step medians, 29.0 days, differs from the median well, 28.1 days, because each step's median can come from a different offset. Quote the programme as the sum of its steps, and keep the well totals beside it as a check.

The best-of-offsets curve takes every step at the fastest offset's time: 23.75 days. No offset achieved it, but each piece of it has been done on this structure. The gap between it and the programme, 5.25 days, is the improvement the team can work towards. The largest single gaps are the 16″ hole, a day, where the losses decide the time, and the 12¼″ hole, three-quarters of a day, where the bit and the ROP do. More than half of the 5.25 days, 2.75, is flat time: a quarter or half a day on each move, casing job, log and completion step.

Split by section, drilling is 14.0 days and flat time 15.0. The two intermediate sections take 14 of the 29 days. Flat time is where programmes are usually optimistic: the rig move, casing, cementing, logging and the completion here add up to more than the drilling. One contingent step is planned and kept out of the total: two days to cure total losses in the Radhuma with cement plugs.

Cost

At 45,000 USD a day the time-related cost is 1.31 million USD and the one-off lines 1.62 million, 2.92 million USD before contingency. The 12¼″ section is the most expensive at 0.74 million, a third of it the 9⅝″ casing. Each extra day anywhere costs at least 45,000 USD, so the 5.25 days between the programme and the best of offsets are worth about 236,000 USD of spread alone.

Three contingencies come from the hazards: curing total losses in the Radhuma (2 days, 130,000 USD), stuck pipe in the Wara (1.5 days, 110,000) and tight hole in the Ahmadi (1 day, 45,000), together 4.5 days and 285,000 USD. The losses and the tight hole, 175,000 USD, go into the AFE's contingency line; the stuck pipe would need a supplement. The two hazards scored High, 12 on a 5 × 5 matrix, are the Radhuma losses and Wara sticking; the Mauddud sour gas scores 8, Medium. The budget itself follows the same structure in the AFE.

Practical guidance

• Read each offset problem as a design input. A tight-hole event means a weight floor; a stuck-pipe event in a depleted sand means a weight ceiling. Put both in the window before picking the mud weight.
• Do not copy a string from another programme without its drift, and do not copy a mud weight without the shoe test it must stay under.
• Plan at the median and target the best. The median keeps the AFE honest; the technical limit shows where the time is.
• Look at flat time. It is half of this programme and is often the easiest half to improve.
• Price the contingencies and decide which belong in the AFE before spud, not after the event.

Where the offsets' time comes from a weighted forecast rather than four wells, see forecasting a well before spud from its offsets; the window itself is the subject of pore pressure, fracture gradient and the mud-weight window.

A checklist for the planning team

• List each offset's casing points, mud weights, shoe tests and NPT events with depth and hours.
• Build the window with trip and surge margins, and mark every loss zone and tight-hole zone on it.
• Set each casing seat where the window must change, and say why in the programme.
• Check every bit against the drift of the string above it, by weight, not just by size.
• Keep every mud weight below the planned shoe test, and plan the test to cover the next section.
• Time each step at the offsets' median and record the fastest offset as the technical limit.
• Cost each section and price each contingency, and decide which are in the AFE.
• Run the full check suite on every revision, not just the first.

How DrillIQ handles it

In DrillIQ the Well Construction Plan module holds the programme: header, tops and targets, casing and mud programmes, hole sections, the sequence with dependencies and contingent steps, pressure data, hazards by depth, the risk register and contingencies, and the completion cost. It reads what the owner modules publish for the well, such as the pore-pressure window, casing and cementing designs, BHAs, hydraulics, the directional plan and well control, and runs the engineering and consistency checks on every change, linking each finding to the row it is about. It draws the time–depth curve and section subtotals, compares the plan with the daily reports and the active AFE, carries revisions and management of change, and prints the Well Program and Front Sheet. The programme and checks above were worked out with the same engine; the cost by section is a hand calculation on its section days.

Book a demo → See all capabilities

References

1. Sharland, P.R., Archer, R., Casey, D.M. et al. Arabian Plate Sequence Stratigraphy. GeoArabia Special Publication 2, 2001.
2. API Specification 5CT: Casing and Tubing. American Petroleum Institute.
3. Bourgoyne, A.T., Millheim, K.K., Chenevert, M.E. and Young, F.S. Applied Drilling Engineering. SPE Textbook Series Vol. 2, 1986.
4. Bond, D.F., Scott, P.W., Page, P.E. and Windham, T.M. Step Change Improvement and High Rate Learning are Delivered by Targeting Technical Limits on Sub-Sea Wells. IADC/SPE 35077, 1996.

About this example. The well in this article is a representative scenario set in typical onshore Kuwait conditions and worked through with DrillIQ's engines. It is not a client's well, and its figures are not field results.