Penetration Rate & Lithology

Measurement of ROP

Rate of penetration is simply a measure of how fast the rig is "making hole." One basic measurement needed for ROP, then, is time while drilling on bottom. The other is distance (or depth) penetrated during that period of active drilling ( Figure 1 , Determination of penetration rate).

Rate of penetration can be quantified in two manners:

Distance per unit of time (e.g., 33 ft/hr, or 10 m/hr)

Time per unit of distance (e.g., 2 min/ft, or 6 min/m)

For computation of ROP, individual distance-per-time intervals must be converted to relative percentages of the total time being averaged. Similarly, each time-per-distance segment must be looked at as a relative percentage of the distance being averaged. If ROP is not calculated correctly, errors result that can affect drilling decisions.

Here is a relatively simple example applied to drilling:

Drilling Time	ROP
3 hours	10 ft/hr
2 hours	5 ft/hr
2 hours	15 ft/hr
2 1/2 hours	4 ft/hr
1/2 hour	20 ft/hr

These figures produce a depth drilled of 90 feet in ten hours (30 + 10 + 30 + 10 + 10 = 90), or 9.0 ft/hr. A straight arithmetic average of drilling rates for the ten hours will show 10.8 ft/hr ([10 + 5 + 15 + 4 + 20]/5 =10.8). This amount of difference (20% error) may be enough to keep a bit on bottom longer than economically wise.

Rate of penetration is generally determined from some combination of the following four methods of measuring drillstring movement — strapping the kelly, drill-rate circular charts, Geolograph strip charts, or mud-logging sensors.

Strapping the Kelly

The oldest method of determining ROP is by marking or strapping the kelly in selected lengths. As each length is drilled, the time required is recorded by hand in a table. As the table is built, so is the base for ROP. The same kelly markings can be used again and again as each pipe connection is made and drilldown continues. This type of physical ROP measurement generally produces a log based on five- or ten-foot increments.

Drill Rate Circular Chart

In this method, a pen or scribe mechanism notes kelly or block movement and plots it on a circular chart that rotates completely once a day ( Figure 2 , Drilling rate and routine operational features indicated on kelly-height circular recorder charts). The apparatus typically is mounted on the rig floor near the driller's station so that the information is readily available to the drill crew and can be manually transcribed for ROP calculations or driller's log notes.

Geolograph Strip Chart

A second common rig-floor mechanical method for recording footage drilled is the Geolograph plot ( Figure 3 , Geolograph strip chart used for recording kelly movement and determining ROP). Like the circular rate curve, the strip chart is cycled daily by a clock mechanism. Drillpipe movements are monitored by a cable attached to the kelly or block; this cable controls the recording scribe in the Geolograph. During routine drilling, the Geolograph is activated by the driller after each connection as the bit goes back on bottom. When the bit drills a predetermined distance and the kelly has been lowered accordingly, generally one foot, a tick mark is automatically scribed on the strip chart. This daily record of information also is transferred manually to logs and tables for ROP calculations.

 

Mud-Logging Sensors

Drilldown measurements used in ROP determinations by most mud-logging operations generally are made by using an idler wheel that senses the movement of the cable driving the Geolograph. When activated during drilling, each rotation of the sensor wheel is transmitted to the mud-logging unit where the signal is automatically digitized, displayed, and stored. This one-foot or one-half-meter signal is then combined with time, or pump strokes, to calculate ROP.

Presentation of ROP Data

Graphically, ROP is always plotted by increments of depth. Because depth (penetration) is vertical on the log, rate is plotted horizontally. Where time-per-distance is used for rate, time increments increase to the right; where distance-per-time is used, distance increments increase to the left ( Figure 4 , Plotting of ROP data-- distance/time vs. time/distance). This causes any increase in ROP to plot to the left in the same fashion as the left-hand track of wireline logs responding to the same parameters that affect ROP (e.g., rock type, porosity).

 

Rate of penetration can be plotted as a bar graph or a continuous line ( Figure 5 , Plotting of ROP data-- bar graph vs. line plot); the horizontal scale can be linear, logarithmic, or proportioned nonlinear ( Figure 6 , Plotting of ROP data-- linear versus non-linear scale). Each type of scale has its advantages and disadvantages. A linear scale, for example, will visually reflect the same amount of ROP change going from 10 to 20 ft/hr as from 50 to 60 ft/hr. However, it is easy to go offscale with a linear grid. Logarithmic or proportioned scales can show visually the
same relative increase of ROP going from 5 to 10 ft/hr as from 10 to 20 ft/hr (a doubling). However, such scales visually emphasize changes at low ROP and diminish them at higher ranges.

Experience shows that the final choice of horizontal scale should be made on the basis of (1) what will keep the expected ROP units on scale, (2) what is common in the geographic area for wireline logs, and (3) what meets the client's practices.

 

 

 

Factors Affecting ROP

Rate of penetration is not a simple reflection of the rock on bottom. A number of factors affect ROP. The principal ones that must be considered when evaluating ROP include the following:

Rock type, porosity, and strength

Bottomhole and bit-face cleaning and differential pressure at bit-rock interface

Bit diameter, type, condition, and jet configuration

Weight on bit and rotary speed

Only the first set of factors is related primarily to rock at depth. The second is more dependent on mud condition and weight; the third is controlled by the bit in use. The fourth set is most related to rigfloor operations and, to a degree, borehole inclination.

It is a simple matter to conclude, then, that an ROP curve can be affected, for example, by rig-floor operations or mud conditions as easily as by rock at depth. An increase in rotary speed can cause the same increase in penetration rate as can a transition to a more drillable lithology. Conversely, an increase in mud weight will slow penetration rate by adversely affecting differential pressure at the cutting interface. Therefore, interpretations concerning ROP should not be made in a vacuum. Consequently, factors such as bit type, mud conditions, hole inclination, and rotary speed are generally recorded or plotted in the ROP track, Track One.

However, once a drilling routine is established with the rig steadily making hole, many of the mud, bit, and rig-floor variables become nearly constant or change systematically. Under these conditions, bottomhole rock can be most influential in determining changes in ROP ( Figure 1 , General effect of rock type on ROP) and thereby be an immediate indication of rock conditions at the bit face.

Use of ROP Data

Under drilling conditions where there is little change in bit, mud, and operating variables, an ROP curve will develop certain characteristics reflecting specific downhole conditions. The most common features in the ROP curve are depicted in Figure 1 (Drilling and geologic conditions often identifiable on an ROP curve, particularly where balanced drilling practices are being observed), and described in Table 1., below. By recognizing such features as compaction trend, drill off-trend, and drilling breaks, some direct interpretation of down hole lithologic and pressure conditions can be made.

The Track One visual plot of ROP can serve as a tool for direct correlation with wireline logs from the same hole or other wells in the area. The most consistent correlations are with porosity logs (e.g., neutron, density, sonic, or in some cases SP and gamma ray). The very best correlation in well-lithified sections usually is obtained with the sonic (Interval Transit Time) log. This does not necessarily hold true, however, in many compacting off-shore Tertiary sections where claystone and mudstone drill as rapidly or more rapidly than sandstone. In older stratigraphic sections, nevertheless, similarities can be so strong between the two logs that it is possible, using normalization techniques, to generate a pseudo-sonic (pseudo-porosity) log that closely duplicates the real thing ( Figure 2 , Comparison of a synthetic (pseudo) sonic log generated by a drilling model with wireline borehole compensated sonic log from the same North Sea section).

The reverse also is possible. Sonic logs from adjoining wells are sometimes used to predict optimum ROP and drill-bit life. In this situation, the projected log can be compared repeatedly to daily formation log ROP and lithology plots to see if bits and drilling procedures are working as was anticipated when preparing the well program.

 

Table 1. Common Features Appearing in ROP Plots

Shale Baseline — The rate at which a uniform lithology (commonly a thick shale interval) is drilled by a single bit over its useful life. (In a carbonate section a limestone baseline may be established for comparative drillability.)

Drilling Break — An abrupt increase in ROP above the baseline average, generally due to change in lithology ( Figure 3 , General effect of rock type on ROP) or sudden increase (fault intersection) in formation pressure.

Reverse Drilling Break — An abrupt decrease in ROP, generally below the baseline average, and generally associated with change in lithology or the presence of a dense "cap" rock.

Drill-off Trend — A gradual or uniform increase in ROP, typically associated with increasing formation pressure (i.e., a transition zone from normal compaction to overpressured subsurface conditions).

Dulling Trend — A noticeable decrease in ROP due to bit wear near the end of its effective life.

Compaction Trend — A long-term decline in ROP (measured across many bit lives) that reflects progressive increase in rock density due to increasing compaction with depth.

Lithology and Shows

The principal data presented on the depth-lithology-show track, Track Two, are observations and depictions of lagged cuttings. Lithologic descriptions generally are recorded in abbreviated form in the right-hand track of the formation log. A list of these abbreviations may be found under References and Additional Information.

Factors Affecting Cuttings

Cuttings, and the shows they may contain, are influenced by four principal factors: lithology
being penetrated, bit characteristics
on bottom, wall stability
of the borehole, and mud system in
use.

Lithology

Although a principal reason for collecting and describing cuttings is to determine the rock types in the geologic section and their downhole changes, changing lithologies create one set of variables. Not all rock types are recovered uniformly. In some cases, such as in poorly consolidated claystones, almost no usable cuttings will be recovered. Such claystones are often made into a "mush" by the bit and incorporated in drilling mud during uphole travel so that they have lost their identity upon arrival at surface. In other cases, only partially representative cuttings will be recovered. Salts, for example, may be dissolved from evaporitic cuttings by water-base muds. In still other situations, although recovery is good, cuttings samples will not accurately represent what is at depth. A friable sandstone, for example, may disaggregate into individual grains so that features like texture, porosity, and staining cannot be accurately observed.

Bit Characteristics

Different bits and different operating conditions lead to different cuttings recovery in the same lithology.

Drag, or "fishtail," bits work by scraping or shearing the bottom of the hole. When used, they generally work best in poor to moderately consolidated clays, silts, and sands where they can peel the rock or make a mush. This shearing action results in poor cuttings at surface. Tricone roller bits generally produce better cuttings for description. Those with chisel-style, longer teeth, which are used in fairly drillable rock, produce good cuttings where tooth penetration and crushing is adequate. Roller bits with carbide buttons, which are generally used to crush harder rock, typically produce cuttings smaller than those from chisel teeth, but these cuttings will be representative of the rock at the bit face. The newly developed polycrystalline diamond compact (PDC) bits cut by a shearing action similar to that of drag bits, but PDCs can handle harder rock; cuttings from PDC bits are generally smaller than those from roller bits and therefore more difficult to describe and evaluate. Diamond bits, including those used for coring, produce the finest cuttings of all in hard rock. The result is that some rock types will be nearly impossible to describe accurately.

Each bit and rock type combination will have some effect, then, on the quality of cuttings reaching surface. And, of course, the manner in which the bit is operated can add another variable. If, for example, the bit face is not cleaned effectively so that "ball up" occurs, very fine cuttings or "mush" may arrive at surface for many rock types. As rotary table speed increases, cuttings will generally get finer. And some downhole drill systems, like turbo-drilling, can leave cuttings and shows with "cooked" appearances.

Wall Stability

Poor wall stability and high formation pressure can lead to cavings and other incursions into the mud system. Cavings can alter the bulk composition of cuttings reaching surface; these must, therefore, be removed by physical (screening) and interpretive (visual) separation during the logging process.

Mud System

A number of influences can be introduced by the particular mud system in use that affect lithologic and show determinations in cuttings. The effect of these must be removed before accurate descriptions can be made. The mud system, for example, may be oil-base or contain hydrocarbons or organic additives; these may obscure in-place petroleum or coal-bearing lithologies in the cuttings. Such contaminants as lost circulation material, recirculating debris, grease, and pipe dope may also be carried in the mud system. Furthermore, an overbalanced mud system can flush oil and gas from permeable rock so that potential reservoir shows are no longer visible during cuttings inspection ( Figure 1 , Borehole and mud conditions affecting cuttings and mud gas before arrival at surface.). In addition, some differential settling and mixing of cuttings can occur during uphole travel in response to mud weight, viscosity, flow patterns, and flow interruptions. Cuttings composition will be smeared accordingly.

 

Presentation of Cuttings Data

Observations recorded relative to lithology will differ with the wishes of the client and the skills of the mud logger. In all cases, however, cuttings and shows will be lagged to their true drilled depth. A percentage plot and an abbreviated description of lithology will be shown in Track Two and Track Five, respectively.

In basic formation logging, the Track Two lithology chart is divided into ten columns, each representing 10% of the cuttings recovered after screening for cavings ( Figure 2 , b). At each depth or sample increment, ten symbols will be drawn, typed, or plotted to indicate the gross lithology of the cuttings as recovered at surface. The symbols used will be from the lithology explanation chart
at the top of the formation log.

A more knowledgeable logger may modify the percentages shown in Track Two to reflect an evaluation of mud contaminants, openhole cavings, and uphole mixing. Such loggers will present their interpretations of cuttings as they are considered to have left bottom rather than how they arrived at surface. Small cavings, for example, will be "interpreted out" by the logger. Track Two in this case may be denoted on a formation log as a corrected cuttings log.

If qualified logging geologists are used, they may produce this corrected cuttings log and supplement it with an additional track showing interpreted true lithology. This will be prepared on the basis of cuttings information combined with drill rate, other drilling parameters, and gas and mud analyses. Formation contacts and lithology changes will be "called" more sharply in this situation.

Oil shows generally are denoted in Track Two by symbols at the depth increments at which they were found in cuttings. A brief numerical rating may be given on the log (often in Track One or Track Five), but most details will be carried in a separate show report identified by number in the show column. Such a show report will contain full notation of such factors as odor, staining, bleeding, reaction in acid, fluorescence, reagent cut, and similar specific tests.

Use of Cuttings Data

Lithologic and oil show data of Track Two generally find four uses: rock type and formation identification, reservoir interval characterization, mud and wireline log correlation, and pressure condition and trend recognition.

Rock Type and Formation Identification

Cuttings are the first hard data on rock type and configuration at depth that can be compared to the data inferred during prospect generation. First appearances of cuttings from different rock types, for example, can provide estimates of formation tops. Refined formation identification can be based on such cuttings factors as specific or characteristic rock type, diagnostic microfossils, and sequence of occurrence. The true vertical depth (TVD) lithology plot of Track Two, then, can be used like any rock section to construct or confirm the stratigraphic column and framework of the area.

Depending on the level of lithologic study provided, a number of other uses can be considered. It may be possible, for example, to infer changes in environment of deposition, as from reef core to back reef; this, in turn may give an indication of whether the active prospect is "on trend" or "shoreward" as proposed in prospect generation. It is also possible in some situations to recognize deformation that has duplicated or omitted stratigraphic intervals or resulted in their apparent thickening or thinning.

Reservoir Interval Characterization

The first documentable indication of porosity and permeability conditions related to oil and gas shows recognized by mud logging comes with inspection of cuttings. In some cases, cuttings characteristics may be adequate to characterize show lithology as relatively tight and of no economic interest. In others, cuttings may give first indications of a probable reservoir.

While drilling, cuttings lithology can be applied in many ways to reservoir considerations. As one example, an anticipated change in cuttings lithology arriving at surface sometimes serves as the basis for a "go ahead" on a planned coring program.

Mud and Wireline Log Correlation

Visual examination of the lithology track, combined with other mud log tracks and wireline logs, finds many uses. Final stratigraphic picks, for example, generally are made after a borehole has been fully logged. Formation tops, which are picked early in a well's history from inspection of cuttings, can always be refined with added information.

Conversely, in order to perform accurate interpretation of wireline log data, lithologic and mineralogic information is required. Although this type of information can be deduced from crossplotting wire-line log data alone, results generally are imprecise, and in mixed or complex lithologies there may be insufficient wireline data for conclusive interpretation. The detailed sample descriptions and mineral identifications available from a complete formation log remove some of the variables so that log response can be clarified and more accurately characterized.

Specialists will use combinations of lithology and show descriptions, mud log hydrocarbon data, and wireline log patterns to complete preliminary reservoir characterizations in many wells. They often utilize this combination of data sources for calculation of expected permeabilities and porosities in show intervals, positioning of sidewall core shots to sample reservoir lithologies, and selection of packer sites for drillstem tests.

Pressure Condition and Trend Recognition

The most recent application of cuttings lithology is for early detection of overpressure transition zones. This application finds most use with balanced drilling programs. From a basic formation log, some pressure information can be deduced from cuttings details. Simple density measurements taken during the description process can show a change in gradient suggestive of transition to overpressure. The fact that cuttings have become larger as penetration rate increases through a uniform lithology can also suggest a lowering of mud overbalance as increasing formation pressure comes into play at the bit-hole bottom interface.

Exercise 1.

Calculate the number of feet drilled during the following twenty-four hour period (assume that pipe length is 30 ft and that connections take 5 minutes), then fill in the blanks below.
0:00-2:30 on bottom following connection completion at 24:00; drilling ahead at 35 ft/hr

2:30-3:15 drilling break, ROP increased to 50 ft/hr

3:15-4:15 gas show, circulated bottoms up

4:15-8:00 no oil show, resumed drilling, ROP decreased to 40 ft/hr

8:00-9:15 short trip into Casing

9:15-11:45 on bottom, drilling ahead, ROP 30 ft/hr

11:45-12:00 no. 1 mud pump down, shift to no. 2

12:00-15:45 drilling ahead; ROP 35 ft/hr

5:45-20:30 increased mud weight; ROP 30 ft/hr

20:30-20:45 reverse drilling break; ROP 15 ft/hr

20:45-21:45 circulate bottoms up; no change

21:45-24:00 trip to check bit

The total depth drilled during the twenty-four hour period was approximately ___ ft.

The bit was on bottom drilling for a period of approximately ___hours ___ minutes (total active drilling time minus connection time).

The penetration rate for the 24-hr period was approximately ___ ft/hr or ___ min/ft.

Solution 1:

Total depth drilled is calculated to be about 573.1 feet.

The total active drilling time was 18 hours, 15 minutes, and the total connection time was (19 x 5) 1 hour, 35 minutes.

Hence, the total on-bottom drilling time was 16 hours 40 minutes. The average ROP was 34.3 ft/hr (572.1/16:40) or 1.75 min/ft (1000 min/573.1 ft; 60 min/34.3 ft).

Exercise 2.

The following factors affect penetration rate; what change has to occur in each case to increase penetration rate?

a. rock drillability

b. mud weight

c. rotary speed

d. bit condition

Solution 2:

a. Greater drillability, greater ROP.

b. Lighter mud weight (lesser differential with formation pressure), greater ROP.

c. Higher rotary speed (through normal speeds), greater ROP.

d. Better bit condition (normally in nearly new condition), greater ROP.

Exercise 3.

What do the following characteristics indicate about formation pressure?

a. shale baseline

b. drilling break

c. drill-off trend

d. dulling trend

Solution 3:

a. If well defined, shale baseline probably indicates normal compaction.

b. If not directly related to lithologic break, the drilling break may indicate crossing abruptly into overpressured section.

c. Most likely a drill-off trend is an indication of drilling through transition zone at top of an overpressured section.

d. If well defined, a dulling trend probably indicates nothing about overpressure; it may help confirm normal compaction.