268:
260:
152:
The differences between loggers and drillers depths are due to different stretch in the drilling string when drilling, and the wire line entered into the bore hole during wireline logging operations. This difference is estimated and referred to as "core shift". A core from a certain drillers depth is
235:
the legal datum offshore
Australia is Lowest Astronomical Tide (LAT) – (Ref. 1 & 2). Note that this requirement in itself can cause difficulties as it is difficult to measure offshore and can vary greatly between locations and even with time. There is, however, an advantage to this convention:
185:
Differential (or relative) depths or thicknesses should generally be specified with at least two components: a unit and a path, plus any eventual specifiers to remove any possible ambiguity. No specifier should ever be left "implicit" or "understood". There are cases where a path is not needed and
156:
Sign
Convention - Depth increases positive in the downward direction. This may seem intuitive but confusion can arise when using certain references while integrating data from different sources. Workers mapping surfaces typically use elevation which, by convention, increases positive in the upward
181:
The acronym TVDSS is commonly used in the oil industry to represent TVD minus the elevation above mean sea level of the depth reference point of the well. The depth reference point is the kelly bushing in the United States and a few other nations, but is the drill floor in most
157:
direction. Be mindful when integrating depth and elevation. For example, shallow wells drilled onshore often encounter reservoir at negative depths when referenced to sea level, mappers would define these same reservoirs at positive elevations when referenced to sea level.
123:
Example: the top of a reservoir may be found at 1,500 mMDRT in a particular well (1,500 m measured depth below the rotary table), which may be equal to 1,492 mTVDMSL (1,492 m true-vertical-depth below mean sea level) after correction for deviations from
228:
Path: common expressions of path are measured depth (MD) – elsewhere often known as along hole depth (AHD) – and true vertical depth (TVD). Note that using TV for true vertical depth is not consistent with the use of MD for measured depth, hence the recommended
284:
in Figure 2 above, the thickness of the reservoir penetrated by the well might be 57 mMD or 42 mTVD, even though the reservoir true stratigraphic thickness in that area (or isopach) might be only 10 m, and its true vertical thickness (isochore), 14 m.
28:
between a point of interest and a reference point or surface. It is the most common method of reference for locations in the well, and therefore, in oil industry speech, "depth" also refers to the location itself.
214:
Special consideration must be given to depth measurement in toe-up laterals (J-profile). In these cases the measured depth will continue to increase while true vertical depth will decrease toward the toe of the
102:
Although it is an intuitive concept, depth in a well is the source of much confusion because it is frequently not specified correctly. Absolute depth should always be specified with three components:
207:
Petrophysicists and drilling operations tend to express depths with reference to the rotary table or the original drill floor; geologists tend to use a common datum such as the mean sea level;
204:
define the "official depths" in a well, and these depths are frequently different from the "drillers' depth", after various corrections, tie-ins, etc., have been applied.
245:
Common references used in operations include: Rotary Table (RT), Drill Floor (DF), Kelly
Bushing (KB), Sea Bottom (SB), Ground Level (GL), Casing Bowl Flange (CBF).
251:
Any combination of unit, path, and reference can be used, as long as they result in fully specified, unambiguous depths. A well may reach to many kilometers.
138:". The "total depth" for the well, core depths and all analysis of core / mud and other materials from the drilling hole are measured in "drillers depth".
146:
71:
and the point of interest. In perfectly vertical wells, the TVD equals the MD; otherwise, the TVD is less than the MD measured from the same datum.
114:
87:
196:
The distinction between "loggers' depth" and "drillers' depth" is becoming blurred due to the increasing use of logs acquired while drilling (
160:
Depth increases in the "down" direction, so an elevation is a negative depth. There is no need to specify above or below the depth reference.
339:
211:
use the mean sea level. This can introduce much confusion when a unit is not specified with all 3 components: unit, path, and reference.
42:, albeit in the opposite direction. However, "depth" in a well is not necessarily measured vertically or along a straight line.
279:
in Figure 1 above, point P1 might be at 3207 mMDRT and 2370 mTVDMSL, while point P2 might be at 2530 mMDRT and 2502 mTVDLAT.
236:
tidal corrections should always be of the same sign (negative depth), i.e. the sea level is always higher than or equal to LAT.
354:
242:
A datum used in the past was Mean Indian Spring Low Water (MISLW). It is comparable to, but not exactly the same as, LAT.
359:
318:
193:
Depths, whether "absolute", "relative", "true", etc., have an intrinsic uncertainty and are never really true.
153:
lined up with a wireline log (loggers depth) and structures in the core are compared with the log and matched.
387:
197:
382:
83:
45:
Because wells are not always drilled vertically, there may be two "depths" for every given point in a
377:
170:
The term "subsea" (SS) by itself should not be used, as it is ambiguous. It could mean: below
186:
in fact should not be specified, because it is defined by the specifier, e.g. isochore (true
17:
8:
299:
62:
135:
34:
175:
200:). At the time of writing, the common practice remains that the petrophysicists or
149:". The loggers depth are typically considered more reliable than the drillers depth.
142:
294:
91:
68:
51:
371:
208:
187:
364:
Log Data
Acquisition and Quality Control, Ph. Theys, 1991, Editions Technip
75:
134:
Well depth values taken during the drilling operation are referred to as "
79:
201:
128:
171:
39:
57:
46:
271:
Fig. 2: Differential depths: reservoir thickness, isochor, isopach
121:
and none of these three components should ever be left implicit.
340:
Sakhalin-1 sets new extended reach drilling record, Rosneft says
267:
259:
25:
113:
and the reference or datum they refer to (e.g. RT for
282:
Specification of a differential depth or a thickness:
239:
A commonly used alternative is Mean Sea Level (MSL).
190:
thickness, independent of well path or inclination).
174:
or bottom, below mean sea level (MSL), below lowest
129:
Depth in a well as used in the oil and gas industry
369:
225:Unit: the usual unit of depth is the metre (m).
360:Seas and Submerged Lands Act 1973 (Australia)
97:
355:Determining Lowest Astronomical Tide (LAT)
74:Common datums used are ground level (GL),
266:
258:
370:
219:
67:, the vertical distance between the
38:is a vertical coordinate related to
277:Specification of an absolute depth:
263:Fig. 1: The specification of depths
110:a path (e.g. MD for measured depth)
13:
14:
399:
348:
56:measured along the path of the
333:
311:
145:operation are referred to as "
1:
305:
7:
288:
141:Well depth values from the
10:
404:
319:"Longest drilled oil well"
254:
107:a unit (e.g. m for metre),
98:The specification of depth
321:. Guinness world records
165:Example: RT = -10 mMDLAT
24:is the distance along a
272:
264:
270:
262:
388:Petroleum production
18:oil and gas industry
300:True vertical depth
63:true vertical depth
383:Energy development
273:
265:
220:Depth in practice
176:astronomical tide
395:
343:
337:
331:
330:
328:
326:
315:
143:wireline loggers
90:(KB or RKB) and
403:
402:
398:
397:
396:
394:
393:
392:
368:
367:
351:
346:
338:
334:
324:
322:
317:
316:
312:
308:
291:
280:
275:
257:
222:
167:
136:driller's depth
131:
100:
22:depth in a well
12:
11:
5:
401:
391:
390:
385:
380:
378:Earth sciences
366:
365:
362:
357:
350:
349:External links
347:
345:
344:
332:
309:
307:
304:
303:
302:
297:
295:Measured depth
290:
287:
256:
253:
249:
248:
247:
246:
243:
240:
237:
230:
226:
221:
218:
217:
216:
212:
205:
194:
191:
183:
179:
163:
162:
161:
158:
154:
150:
147:logger's depth
139:
130:
127:
119:
118:
111:
108:
99:
96:
92:mean sea level
52:measured depth
9:
6:
4:
3:
2:
400:
389:
386:
384:
381:
379:
376:
375:
373:
363:
361:
358:
356:
353:
352:
341:
336:
320:
314:
310:
301:
298:
296:
293:
292:
286:
283:
278:
269:
261:
252:
244:
241:
238:
234:
233:
231:
227:
224:
223:
213:
210:
209:geophysicists
206:
203:
199:
195:
192:
189:
188:stratigraphic
184:
180:
177:
173:
169:
168:
166:
159:
155:
151:
148:
144:
140:
137:
133:
132:
126:
125:
116:
112:
109:
106:
105:
104:
95:
93:
89:
88:kelly bushing
85:
81:
77:
72:
70:
66:
64:
59:
55:
53:
48:
43:
41:
37:
36:
30:
27:
23:
19:
335:
323:. Retrieved
313:
281:
276:
274:
250:
164:
122:
120:
115:rotary table
101:
84:Rotary table
76:drilling rig
73:
61:
50:
44:
33:
31:
21:
15:
232:Reference:
178:(LAT), etc.
372:Categories
306:References
202:geologists
60:, and the
32:Strictly,
215:wellbore.
172:sea floor
124:vertical.
40:elevation
325:26 April
289:See also
58:borehole
47:wellbore
255:Figures
182:places.
94:(MSL).
78:floor (
49:: the
16:In the
342:, 2015
86:(RT),
69:datum
65:(TVD)
35:depth
327:2017
229:TVD.
54:(MD)
26:well
198:LWD
82:),
374::
117:),
80:DF
20:,
329:.
Text is available under the Creative Commons Attribution-ShareAlike License. Additional terms may apply.
