Well Planning

3. Well Planning

Habiburrohman abdullah

Well Planning

• Pore Pressure and Fracture Gradient

• Casing Depth Selection

Types of Well Planning

• Exploration well planning.

• Development well planning.

• Completion/workover programme planning.

Well Planning

• Close interaction between Drilling and the Exploration, Petroleum Engineering, Design and Construction Departments is vital to optimise total project costs. Rig/platform interaction, well reach and well design options should be fully evaluated in the conceptual stage. All platform options and well design options should be considered and all possibilities for reducing costs should be examined.

Typical Well Configuration

Typical well configuration between exploration wells and development well

Figure 1: Well Configuration

Pore Pressure and Fracture Gradient

Parameters like pore pressure and formation strength determine aspects like:

1. Choice of mud weight profile,

2. Determination of casing setting depths,

3. Design of optimal casing strings,

4. Selection of the drill bit,

5. Cementing additives and procedures.

RKB & MSL

• Within drilling engineering, the drilling floor or rotary table (RKB) is the most often used reference depth.

• Geologists and geophysicists generally prefer to use their data in reference to ground floor or mean sea level (MSL).

Well Depth Guidance

• Correct RKB to MSL reference

– ……………………………..(1)

• Convert MSL data to RKB

…………….………………..(2)

RKBRKBMSL hD

Ddd

D

hDdd RKBMSLRKB

Wildcat to Development Well Correlation

• Another common problem is when data referenced to one RKB (e.g. rig used to drill the wildcat well) has to be applied for further/later calculations (e.g. drilling development wells from a production platform). Here the data have to be corrected from RKB1 to RKB2 .

• Correct from RKB1 to RKB2:

……………………………..(3)

D

hDdd RKBRKB

12

where:

• D [m or ft] ... total depth of point of interest in reference to RKB

• hRKB [m or ft] ... height of RKB above MSL

• Δh [m or ft] ... difference of elevation of RKB1 to RKB2

Wildcat to Development Well Correlation

Geology Prediction

• Normally when a well is to be drilled, the drilling engineer is supplied from the geology department with a sequence of predicted subsurface formations, their characteristics and markers, as well as knowledge about where special care has to be taken.

Figure 2: Typical geological profile to plan a well

Pore pressure Prediction-Hydrostatic Pressure

• By definition, a hydrostatic pressure is developed due to the own weight of a fluid at a certain depth.

……………………………..(4)

• Or in field units:

……………………………………..(5)

hhgp 81.9

Dp fl052.0


Where

• ρfl [ppg] ... density of the fluid causing hydrostatic pressure

• ρ [kg/m3] ... average fluid density• D [ft] ... depth at which hydrostatic pressure occurs (TVD)• h [m] ... vertical height of column of liquid• p [psi] ... hydrostatic pressure• g [m/s2] ... acceleration due to gravity


When the burial depth increases, the overlaying pressure (overburden stress) increases. This decreases the pore space between the grains and thus the porosity of the formation.

Figure 3: Porosity profile with increasing depth


• When drilling a well, formations are often encountered that are under a different pressure regime. These formations are named to be “abnormally pressured”. Abnormal pressures can be positive (actual formation pressures are higher than hydrostatic pressure) or negative (actual formation pressures are lower than hydrostatic pressure). Sometimes the term “subnormal pressure” is used when the formation pressure is lower than the hydrostatic one.


• Some mechanisms that lead to abnormally pressured formations are:

• 1. Compaction effects,• 2. Artesian System• 3. Uplift• 4. Salt Beds• 5. Differential density effects (Osmosis)• 6. Salt Domes


Compaction effects

A sealing mechanism must be present to trap abnormal pressure environment. The common sealing mechanism is a shale section. The shale reduces normal fluid escape, causing undercompaction and abnormal fluid pressure.

….(6)

Where:

D1 = depth of interest below the barrier, ft.

DB = depth of the barrier, i.e., low permeability section, ft

P = formation pressure at D1, psi

)(/0.1/465.0 1 BB DDftpsiDftpsiP


• While burying of the sediments, formation water is expelled with increasing depth and temperatures due to reduction in pore space and diagenesis of the rock materials.

• As long as the permeability and the effective porosity of the rock is high enough so that the formation water can escape as quickly as the natural compaction takes place, the formations are normally pressured. The (vertical) pressures acting inside formations can be modelled as:


……………………………………(7)

Where :

σob [psi] ... overburden stress

σz [psi] ... vertical stress supported by the grain-to-grain connections

p [psi] ... formation pore pressure

pzob


• The bulk density [ppg] of a formation is estimated by equation:

……………………………….(8)

where:

ρg [ppg] ... grain density

ρfl [ppg] ... formation fluid density

[1] ... total porosity of the formation

flgb )1(

Casing Depth Selection

Type of casing :

• Conductor casing

• Surface casing

• Intermediate casing

• Production casing.

Casing Depth SelectionA. Conductor casing

Applications:

- to provide a fluid conduit from the bit to the surface.

- to minimize hole caving.

B. Surface Casing Applications:

- cover freshwater sands

- maintain hole integrity by preventing caving

- minimize lost circulation into shallow, permeable zones

- cover weak zone that are incompetent to control kick-imposed pressure

- provide a means of attaching of BOP

- support the weight of all casings run below the surface pipe

Casing Depth Selection

C. Intermediate casingApplications:

- used to isolate salt zones or zones that cause hole problems, such as heaving and sloughing shales.

D. Production casing.Applications:

- isolate the producing zones from the other formations.

- provide a work shaft of a known diameter to the pay zones.

- protect the production tubing equipment.

Setting Depth Design Procedures

• Casing seat depth are directly affected by geological conditions. In some cases, the prime criterion for selecting casing seats is to cover exposed, severe lost circulation zones, differential sticking problems, pressure depletion, controlling abnormal pressure and preventing their exposure to weaker shallow zones.

Setting Depth Selection for Intermediate and Deeper Strings

Criteria:

1.Let the mud weight control formation pressure without fracturing shallow formations (this considerations are made from bottom to up)

2.Differential pressure sticking determination to be made to verify the casing string will become stuck when running it to the well (this considerations are made from top to bottom)

Setting Depth Selection for Intermediate and Deeper Strings

General values for the amount of differential pressure that can be tolerated before sticking occurs:

- Normal pressure zones: 2,000 – 2,300 psi

- Abnormal pressure zones: 3,000 – 3,300 psi

Setting Depth Selection for Intermediate and Deeper StringsEquation to determine the new intermediate depth if sticking is a concern:

…………………………..(9)

Where :

MW = mud weight, lb/gal

D = depth to deepest normal zone, ft

P = differential pressure, psi

DMWP 052.09

Setting Depth Selection for Intermediate and Deeper StringsAn arbitrary limit of 2,000 – 2,300 psi is normally used for P. The mud weight, MW from equation (9) can be used to locate the depth where the P value will exist:

……………………………………….(10)

Where :

MW = mud weight, lb/gal

TM = trip margin, lb/gal

P = formation pressure, lb/gal

PTMMW

Surface Casing Depth Selection

The most problem at this section is a “kick”. Kick imposed equivalent mud weights are the cause for most underground blowouts.

………………….(11)

Where :

EMWkick = equivalent mud weight at the depth of interest, lb/gal

total depth = deepest interval, ft

depth of interest = ft

M = incremental kick mud weight increase, lb/gal

OMW = original mud weight, lb/gal

OMWMerestofdepth

depthtotalEMWkick

int

END